diff --git a/backend/alembic/versions/002_add_invite_codes.py b/backend/alembic/versions/002_add_invite_codes.py
new file mode 100644
index 00000000..630bd4b6
--- /dev/null
+++ b/backend/alembic/versions/002_add_invite_codes.py
@@ -0,0 +1,52 @@
+"""Add invite codes
+
+Revision ID: 002
+Revises: 7e00fa3c75c9
+Create Date: 2026-02-01
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = '002'
+down_revision: Union[str, None] = '7e00fa3c75c9'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Create invite_codes table
+    op.create_table(
+        'invite_codes',
+        sa.Column('id', postgresql.UUID(as_uuid=True), primary_key=True),
+        sa.Column('code', sa.String(16), nullable=False, unique=True, index=True),
+        sa.Column('created_by_id', postgresql.UUID(as_uuid=True), sa.ForeignKey('users.id'), nullable=False),
+        sa.Column('used_by_id', postgresql.UUID(as_uuid=True), sa.ForeignKey('users.id'), nullable=True),
+        sa.Column('expires_at', sa.DateTime(timezone=True), nullable=True),
+        sa.Column('note', sa.String(255), nullable=True),
+        sa.Column('created_at', sa.DateTime(timezone=True), nullable=False, server_default=sa.text('now()')),
+        sa.Column('used_at', sa.DateTime(timezone=True), nullable=True),
+    )
+
+    # Add invite_code_id FK to users table
+    op.add_column('users', sa.Column('invite_code_id', postgresql.UUID(as_uuid=True), nullable=True))
+    op.create_foreign_key(
+        'fk_users_invite_code_id',
+        'users',
+        'invite_codes',
+        ['invite_code_id'],
+        ['id']
+    )
+
+
+def downgrade() -> None:
+    # Remove FK and column from users
+    op.drop_constraint('fk_users_invite_code_id', 'users', type_='foreignkey')
+    op.drop_column('users', 'invite_code_id')
+
+    # Drop invite_codes table
+    op.drop_table('invite_codes')
diff --git a/backend/app/api/endpoints/auth.py b/backend/app/api/endpoints/auth.py
index cd7ef183..97b79708 100644
--- a/backend/app/api/endpoints/auth.py
+++ b/backend/app/api/endpoints/auth.py
@@ -5,6 +5,7 @@ from fastapi.security import OAuth2PasswordRequestForm
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 
+from app.core.config import settings
 from app.core.database import get_db
 from app.core.security import (
     verify_password,
@@ -14,6 +15,7 @@ from app.core.security import (
     decode_token
 )
 from app.models.user import User
+from app.models.invite_code import InviteCode
 from app.schemas.user import UserCreate, UserResponse, UserLogin
 from app.schemas.token import Token
 from app.api.deps import get_current_user
@@ -27,6 +29,39 @@ async def register(
     db: Annotated[AsyncSession, Depends(get_db)]
 ):
     """Register a new user."""
+    # Validate invite code if required
+    invite_code_record = None
+    if settings.REQUIRE_INVITE_CODE:
+        if not user_data.invite_code:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code is required"
+            )
+
+        # Look up invite code (case-insensitive)
+        result = await db.execute(
+            select(InviteCode).where(InviteCode.code == user_data.invite_code.upper())
+        )
+        invite_code_record = result.scalar_one_or_none()
+
+        if not invite_code_record:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invalid invite code"
+            )
+
+        if invite_code_record.is_used:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code has already been used"
+            )
+
+        if invite_code_record.is_expired:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code has expired"
+            )
+
     # Check if email already exists
     result = await db.execute(select(User).where(User.email == user_data.email))
     existing_user = result.scalar_one_or_none()
@@ -41,9 +76,16 @@ async def register(
         email=user_data.email,
         password_hash=get_password_hash(user_data.password),
         name=user_data.name,
-        role=user_data.role  # Use role from request (defaults to "engineer")
+        role=user_data.role,
+        invite_code_id=invite_code_record.id if invite_code_record else None
     )
     db.add(new_user)
+
+    # Mark invite code as used
+    if invite_code_record:
+        invite_code_record.used_by_id = new_user.id
+        invite_code_record.used_at = datetime.now(timezone.utc)
+
     await db.commit()
     await db.refresh(new_user)
 
diff --git a/backend/app/api/endpoints/invite.py b/backend/app/api/endpoints/invite.py
new file mode 100644
index 00000000..9784b0df
--- /dev/null
+++ b/backend/app/api/endpoints/invite.py
@@ -0,0 +1,96 @@
+from datetime import datetime, timezone
+from typing import Annotated
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
+
+from app.core.database import get_db
+from app.models.user import User
+from app.models.invite_code import InviteCode
+from app.schemas.invite_code import InviteCodeCreate, InviteCodeResponse, InviteCodeValidation
+from app.api.deps import require_admin
+
+router = APIRouter(prefix="/invites", tags=["invites"])
+
+
+@router.post("", response_model=InviteCodeResponse, status_code=status.HTTP_201_CREATED)
+async def create_invite_code(
+    invite_data: InviteCodeCreate,
+    current_user: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)]
+):
+    """Create a new invite code. Admin only."""
+    invite_code = InviteCode(
+        created_by_id=current_user.id,
+        expires_at=invite_data.expires_at,
+        note=invite_data.note
+    )
+    db.add(invite_code)
+    await db.commit()
+    await db.refresh(invite_code)
+
+    return invite_code
+
+
+@router.get("", response_model=list[InviteCodeResponse])
+async def list_invite_codes(
+    current_user: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)]
+):
+    """List all invite codes. Admin only."""
+    result = await db.execute(
+        select(InviteCode).order_by(InviteCode.created_at.desc())
+    )
+    invite_codes = result.scalars().all()
+    return invite_codes
+
+
+@router.delete("/{code}", status_code=status.HTTP_204_NO_CONTENT)
+async def revoke_invite_code(
+    code: str,
+    current_user: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)]
+):
+    """Revoke (delete) an invite code. Admin only."""
+    result = await db.execute(
+        select(InviteCode).where(InviteCode.code == code)
+    )
+    invite_code = result.scalar_one_or_none()
+
+    if not invite_code:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Invite code not found"
+        )
+
+    if invite_code.is_used:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Cannot revoke a used invite code"
+        )
+
+    await db.delete(invite_code)
+    await db.commit()
+
+
+@router.get("/validate/{code}", response_model=InviteCodeValidation)
+async def validate_invite_code(
+    code: str,
+    db: Annotated[AsyncSession, Depends(get_db)]
+):
+    """Check if an invite code is valid. Public endpoint for UX."""
+    result = await db.execute(
+        select(InviteCode).where(InviteCode.code == code.upper())
+    )
+    invite_code = result.scalar_one_or_none()
+
+    if not invite_code:
+        return InviteCodeValidation(valid=False, message="Invalid invite code")
+
+    if invite_code.is_used:
+        return InviteCodeValidation(valid=False, message="Invite code has already been used")
+
+    if invite_code.is_expired:
+        return InviteCodeValidation(valid=False, message="Invite code has expired")
+
+    return InviteCodeValidation(valid=True, message="Invite code is valid")
diff --git a/backend/app/api/router.py b/backend/app/api/router.py
index e1c5936c..7779bbbe 100644
--- a/backend/app/api/router.py
+++ b/backend/app/api/router.py
@@ -1,8 +1,9 @@
 from fastapi import APIRouter
-from app.api.endpoints import auth, trees, sessions
+from app.api.endpoints import auth, trees, sessions, invite
 
 api_router = APIRouter()
 
 api_router.include_router(auth.router)
 api_router.include_router(trees.router)
 api_router.include_router(sessions.router)
+api_router.include_router(invite.router)
diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index cd858f87..7be1b8a3 100644
--- a/backend/app/core/config.py
+++ b/backend/app/core/config.py
@@ -38,6 +38,9 @@ class Settings(BaseSettings):
     # Security
     BCRYPT_ROUNDS: int = 12
 
+    # Registration
+    REQUIRE_INVITE_CODE: bool = True  # Set to False to allow open registration
+
     # CORS - set FRONTEND_URL in production (e.g., https://patherly.up.railway.app)
     CORS_ORIGINS: list[str] = ["http://localhost:3000", "http://localhost:5173", "http://localhost:5174"]
     FRONTEND_URL: Optional[str] = None
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index 9c752f54..2b91354c 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -3,5 +3,6 @@ from .team import Team
 from .tree import Tree
 from .session import Session
 from .attachment import Attachment
+from .invite_code import InviteCode
 
-__all__ = ["User", "Team", "Tree", "Session", "Attachment"]
+__all__ = ["User", "Team", "Tree", "Session", "Attachment", "InviteCode"]
diff --git a/backend/app/models/invite_code.py b/backend/app/models/invite_code.py
new file mode 100644
index 00000000..4f2c6615
--- /dev/null
+++ b/backend/app/models/invite_code.py
@@ -0,0 +1,86 @@
+import uuid
+import secrets
+import string
+from datetime import datetime, timezone
+from typing import Optional
+from sqlalchemy import String, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+
+def generate_invite_code() -> str:
+    """Generate an 8-character alphanumeric invite code."""
+    alphabet = string.ascii_uppercase + string.digits
+    # Remove confusing characters: 0, O, I, 1
+    alphabet = alphabet.replace("0", "").replace("O", "").replace("I", "").replace("1", "")
+    return "".join(secrets.choice(alphabet) for _ in range(8))
+
+
+class InviteCode(Base):
+    __tablename__ = "invite_codes"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    code: Mapped[str] = mapped_column(
+        String(16),
+        unique=True,
+        nullable=False,
+        index=True,
+        default=generate_invite_code
+    )
+    created_by_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=False
+    )
+    used_by_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=True
+    )
+    expires_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True
+    )
+    note: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
+    )
+    used_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True
+    )
+
+    # Relationships
+    created_by: Mapped["User"] = relationship(
+        "User",
+        foreign_keys=[created_by_id],
+        backref="created_invite_codes"
+    )
+    used_by: Mapped[Optional["User"]] = relationship(
+        "User",
+        foreign_keys=[used_by_id],
+        backref="used_invite_code"
+    )
+
+    @property
+    def is_used(self) -> bool:
+        """Check if the invite code has been used."""
+        return self.used_by_id is not None
+
+    @property
+    def is_expired(self) -> bool:
+        """Check if the invite code has expired."""
+        if self.expires_at is None:
+            return False
+        return datetime.now(timezone.utc) > self.expires_at
+
+    @property
+    def is_valid(self) -> bool:
+        """Check if the invite code is valid (not used and not expired)."""
+        return not self.is_used and not self.is_expired
diff --git a/backend/app/models/user.py b/backend/app/models/user.py
index 04e7844a..37b411fa 100644
--- a/backend/app/models/user.py
+++ b/backend/app/models/user.py
@@ -24,6 +24,11 @@ class User(Base):
         ForeignKey("teams.id"),
         nullable=True
     )
+    invite_code_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("invite_codes.id"),
+        nullable=True
+    )
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc)
diff --git a/backend/app/schemas/invite_code.py b/backend/app/schemas/invite_code.py
new file mode 100644
index 00000000..1e812c82
--- /dev/null
+++ b/backend/app/schemas/invite_code.py
@@ -0,0 +1,34 @@
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+from pydantic import BaseModel, Field
+
+
+class InviteCodeCreate(BaseModel):
+    """Schema for creating a new invite code."""
+    expires_at: Optional[datetime] = Field(None, description="Optional expiration time")
+    note: Optional[str] = Field(None, max_length=255, description="Note about who this code is for")
+
+
+class InviteCodeResponse(BaseModel):
+    """Schema for invite code response."""
+    id: UUID
+    code: str
+    created_by_id: UUID
+    used_by_id: Optional[UUID] = None
+    expires_at: Optional[datetime] = None
+    note: Optional[str] = None
+    created_at: datetime
+    used_at: Optional[datetime] = None
+    is_used: bool
+    is_expired: bool
+    is_valid: bool
+
+    class Config:
+        from_attributes = True
+
+
+class InviteCodeValidation(BaseModel):
+    """Schema for invite code validation response."""
+    valid: bool
+    message: str
diff --git a/backend/app/schemas/user.py b/backend/app/schemas/user.py
index 2383afd0..3e10f6c0 100644
--- a/backend/app/schemas/user.py
+++ b/backend/app/schemas/user.py
@@ -12,6 +12,7 @@ class UserBase(BaseModel):
 class UserCreate(UserBase):
     password: str = Field(..., min_length=10, description="Password must be at least 10 characters")
     role: str = Field(default="engineer", description="User role: admin, engineer, or viewer")
+    invite_code: Optional[str] = Field(None, description="Invite code for registration (required when invite system is enabled)")
 
 
 class UserUpdate(BaseModel):
diff --git a/frontend/src/api/index.ts b/frontend/src/api/index.ts
index 09bfba47..c58253bf 100644
--- a/frontend/src/api/index.ts
+++ b/frontend/src/api/index.ts
@@ -2,3 +2,4 @@ export { default as apiClient } from './client'
 export { default as authApi } from './auth'
 export { default as treesApi } from './trees'
 export { default as sessionsApi } from './sessions'
+export { default as inviteApi } from './invite'
diff --git a/frontend/src/api/invite.ts b/frontend/src/api/invite.ts
new file mode 100644
index 00000000..f548321e
--- /dev/null
+++ b/frontend/src/api/invite.ts
@@ -0,0 +1,11 @@
+import apiClient from './client'
+import type { InviteCodeValidation } from '@/types'
+
+export const inviteApi = {
+  async validateCode(code: string): Promise<InviteCodeValidation> {
+    const response = await apiClient.get<InviteCodeValidation>(`/invites/validate/${code}`)
+    return response.data
+  },
+}
+
+export default inviteApi
diff --git a/frontend/src/pages/RegisterPage.tsx b/frontend/src/pages/RegisterPage.tsx
index d9c3adaf..03d421e3 100644
--- a/frontend/src/pages/RegisterPage.tsx
+++ b/frontend/src/pages/RegisterPage.tsx
@@ -1,23 +1,55 @@
 import { useState } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
 import { useAuthStore } from '@/store/authStore'
+import { inviteApi } from '@/api'
 import { cn } from '@/lib/utils'
 
 export function RegisterPage() {
   const navigate = useNavigate()
   const { register, isLoading, error, clearError } = useAuthStore()
 
+  const [inviteCode, setInviteCode] = useState('')
+  const [inviteCodeStatus, setInviteCodeStatus] = useState<'idle' | 'checking' | 'valid' | 'invalid'>('idle')
+  const [inviteCodeMessage, setInviteCodeMessage] = useState('')
   const [name, setName] = useState('')
   const [email, setEmail] = useState('')
   const [password, setPassword] = useState('')
   const [confirmPassword, setConfirmPassword] = useState('')
   const [localError, setLocalError] = useState('')
 
+  const validateInviteCode = async (code: string) => {
+    if (!code.trim()) {
+      setInviteCodeStatus('idle')
+      setInviteCodeMessage('')
+      return
+    }
+
+    setInviteCodeStatus('checking')
+    try {
+      const result = await inviteApi.validateCode(code.trim())
+      setInviteCodeStatus(result.valid ? 'valid' : 'invalid')
+      setInviteCodeMessage(result.message)
+    } catch {
+      setInviteCodeStatus('invalid')
+      setInviteCodeMessage('Failed to validate invite code')
+    }
+  }
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
     setLocalError('')
     clearError()
 
+    if (!inviteCode.trim()) {
+      setLocalError('Invite code is required')
+      return
+    }
+
+    if (inviteCodeStatus !== 'valid') {
+      setLocalError('Please enter a valid invite code')
+      return
+    }
+
     if (!name || !email || !password) {
       setLocalError('Please fill in all fields')
       return
@@ -34,7 +66,7 @@ export function RegisterPage() {
     }
 
     try {
-      await register({ email, password, name })
+      await register({ email, password, name, invite_code: inviteCode.trim() })
       navigate('/trees', { replace: true })
     } catch {
       // Error is set in the store
@@ -57,6 +89,43 @@ export function RegisterPage() {
               </div>
             )}
 
+            <div>
+              <label htmlFor="inviteCode" className="block text-sm font-medium text-foreground">
+                Invite code
+              </label>
+              <input
+                id="inviteCode"
+                name="inviteCode"
+                type="text"
+                required
+                value={inviteCode}
+                onChange={(e) => {
+                  setInviteCode(e.target.value.toUpperCase())
+                  setInviteCodeStatus('idle')
+                }}
+                onBlur={(e) => validateInviteCode(e.target.value)}
+                className={cn(
+                  'mt-1 block w-full rounded-md border bg-background px-3 py-2 font-mono tracking-wider',
+                  'text-foreground placeholder:text-muted-foreground',
+                  'focus:outline-none focus:ring-1',
+                  inviteCodeStatus === 'valid' && 'border-green-500 focus:border-green-500 focus:ring-green-500',
+                  inviteCodeStatus === 'invalid' && 'border-destructive focus:border-destructive focus:ring-destructive',
+                  inviteCodeStatus === 'idle' && 'border-input focus:border-primary focus:ring-primary',
+                  inviteCodeStatus === 'checking' && 'border-input focus:border-primary focus:ring-primary'
+                )}
+                placeholder="ABCD1234"
+              />
+              {inviteCodeStatus === 'checking' && (
+                <p className="mt-1 text-xs text-muted-foreground">Validating...</p>
+              )}
+              {inviteCodeStatus === 'valid' && (
+                <p className="mt-1 text-xs text-green-600">{inviteCodeMessage}</p>
+              )}
+              {inviteCodeStatus === 'invalid' && (
+                <p className="mt-1 text-xs text-destructive">{inviteCodeMessage}</p>
+              )}
+            </div>
+
             <div>
               <label htmlFor="name" className="block text-sm font-medium text-foreground">
                 Full name
diff --git a/frontend/src/types/index.ts b/frontend/src/types/index.ts
index 65d2dda7..f21606b5 100644
--- a/frontend/src/types/index.ts
+++ b/frontend/src/types/index.ts
@@ -2,6 +2,7 @@ export * from './user'
 export * from './auth'
 export * from './tree'
 export * from './session'
+export * from './invite'
 
 // API response wrapper types
 export interface PaginatedResponse<T> {
diff --git a/frontend/src/types/invite.ts b/frontend/src/types/invite.ts
new file mode 100644
index 00000000..cd40946a
--- /dev/null
+++ b/frontend/src/types/invite.ts
@@ -0,0 +1,4 @@
+export interface InviteCodeValidation {
+  valid: boolean
+  message: string
+}
diff --git a/frontend/src/types/user.ts b/frontend/src/types/user.ts
index 33697ba2..7fdf490e 100644
--- a/frontend/src/types/user.ts
+++ b/frontend/src/types/user.ts
@@ -15,6 +15,7 @@ export interface UserCreate {
   password: string
   name: string
   role?: UserRole
+  invite_code?: string
 }
 
 export interface UserLogin {