diff --git a/src/app/editor/[projectId]/FlowchartEditor.tsx b/src/app/editor/[projectId]/FlowchartEditor.tsx
index ade31fe..2ba1cab 100644
--- a/src/app/editor/[projectId]/FlowchartEditor.tsx
+++ b/src/app/editor/[projectId]/FlowchartEditor.tsx
@@ -1,6 +1,6 @@
 'use client'
 
-import React, { useCallback, useMemo, useState } from 'react'
+import React, { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import ReactFlow, {
   Background,
   BackgroundVariant,
@@ -27,10 +27,12 @@ import ConditionEditor from '@/components/editor/ConditionEditor'
 import ExportValidationModal, { type ValidationIssue } from '@/components/editor/ExportValidationModal'
 import { EditorProvider } from '@/components/editor/EditorContext'
 import Toast from '@/components/Toast'
+import { RealtimeConnection, type ConnectionState } from '@/lib/collaboration/realtime'
 import type { FlowchartData, FlowchartNode, FlowchartEdge, Character, Variable, Condition } from '@/types/flowchart'
 
 type FlowchartEditorProps = {
   projectId: string
+  userId: string
   initialData: FlowchartData
   needsMigration?: boolean
 }
@@ -202,7 +204,7 @@ function computeMigration(initialData: FlowchartData, shouldMigrate: boolean) {
 }
 
 // Inner component that uses useReactFlow hook
-function FlowchartEditorInner({ projectId, initialData, needsMigration }: FlowchartEditorProps) {
+function FlowchartEditorInner({ projectId, userId, initialData, needsMigration }: FlowchartEditorProps) {
   // Define custom node types - memoized to prevent re-renders
   const nodeTypes: NodeTypes = useMemo(
     () => ({
@@ -232,6 +234,22 @@ function FlowchartEditorInner({ projectId, initialData, needsMigration }: Flowch
   const [toastMessage, setToastMessage] = useState<string | null>(migratedData.toastMessage)
   const [validationIssues, setValidationIssues] = useState<ValidationIssue[] | null>(null)
   const [warningNodeIds, setWarningNodeIds] = useState<Set<string>>(new Set())
+  const [connectionState, setConnectionState] = useState<ConnectionState>('disconnected')
+  const realtimeRef = useRef<RealtimeConnection | null>(null)
+
+  // Connect to Supabase Realtime channel on mount, disconnect on unmount
+  useEffect(() => {
+    const connection = new RealtimeConnection(projectId, userId, {
+      onConnectionStateChange: setConnectionState,
+    })
+    realtimeRef.current = connection
+    connection.connect()
+
+    return () => {
+      connection.disconnect()
+      realtimeRef.current = null
+    }
+  }, [projectId, userId])
 
   const handleAddCharacter = useCallback(
     (name: string, color: string): string => {
@@ -511,6 +529,7 @@ function FlowchartEditorInner({ projectId, initialData, needsMigration }: Flowch
           onExport={handleExport}
           onImport={handleImport}
           onProjectSettings={() => setShowSettings(true)}
+          connectionState={connectionState}
         />
         <div className="flex-1">
           <ReactFlow
diff --git a/src/app/editor/[projectId]/page.tsx b/src/app/editor/[projectId]/page.tsx
index 7c5f64a..ce16cb0 100644
--- a/src/app/editor/[projectId]/page.tsx
+++ b/src/app/editor/[projectId]/page.tsx
@@ -73,6 +73,7 @@ export default async function EditorPage({ params }: PageProps) {
       <div className="flex-1">
         <FlowchartEditor
           projectId={project.id}
+          userId={user.id}
           initialData={flowchartData}
           needsMigration={needsMigration}
         />
diff --git a/src/components/editor/Toolbar.tsx b/src/components/editor/Toolbar.tsx
index 81c2c45..3161530 100644
--- a/src/components/editor/Toolbar.tsx
+++ b/src/components/editor/Toolbar.tsx
@@ -1,5 +1,7 @@
 'use client'
 
+import type { ConnectionState } from '@/lib/collaboration/realtime'
+
 type ToolbarProps = {
   onAddDialogue: () => void
   onAddChoice: () => void
@@ -8,6 +10,21 @@ type ToolbarProps = {
   onExport: () => void
   onImport: () => void
   onProjectSettings: () => void
+  connectionState?: ConnectionState
+}
+
+const connectionLabel: Record<ConnectionState, string> = {
+  connecting: 'Connecting…',
+  connected: 'Connected',
+  disconnected: 'Disconnected',
+  reconnecting: 'Reconnecting…',
+}
+
+const connectionColor: Record<ConnectionState, string> = {
+  connecting: 'bg-yellow-400',
+  connected: 'bg-green-400',
+  disconnected: 'bg-red-400',
+  reconnecting: 'bg-yellow-400',
 }
 
 export default function Toolbar({
@@ -18,6 +35,7 @@ export default function Toolbar({
   onExport,
   onImport,
   onProjectSettings,
+  connectionState,
 }: ToolbarProps) {
   return (
     <div className="flex items-center justify-between border-b border-zinc-200 bg-zinc-50 px-4 py-2 dark:border-zinc-700 dark:bg-zinc-800">
@@ -46,6 +64,14 @@ export default function Toolbar({
       </div>
 
       <div className="flex items-center gap-2">
+        {connectionState && (
+          <div className="flex items-center gap-1.5 mr-2" title={connectionLabel[connectionState]}>
+            <span className={`inline-block h-2.5 w-2.5 rounded-full ${connectionColor[connectionState]}${connectionState === 'reconnecting' ? ' animate-pulse' : ''}`} />
+            <span className="text-xs text-zinc-500 dark:text-zinc-400">
+              {connectionLabel[connectionState]}
+            </span>
+          </div>
+        )}
         <button
           onClick={onProjectSettings}
           className="rounded border border-zinc-300 bg-white px-3 py-1.5 text-sm font-medium text-zinc-700 hover:bg-zinc-50 focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 dark:border-zinc-600 dark:bg-zinc-700 dark:text-zinc-200 dark:hover:bg-zinc-600 dark:focus:ring-offset-zinc-800"
diff --git a/src/lib/collaboration/realtime.ts b/src/lib/collaboration/realtime.ts
new file mode 100644
index 0000000..9be5547
--- /dev/null
+++ b/src/lib/collaboration/realtime.ts
@@ -0,0 +1,167 @@
+import { createClient } from '@/lib/supabase/client'
+import type { RealtimeChannel } from '@supabase/supabase-js'
+
+export type ConnectionState = 'connecting' | 'connected' | 'disconnected' | 'reconnecting'
+
+type RealtimeCallbacks = {
+  onConnectionStateChange: (state: ConnectionState) => void
+  onPresenceSync?: (presences: Record<string, unknown[]>) => void
+}
+
+const HEARTBEAT_INTERVAL_MS = 30_000
+const RECONNECT_BASE_DELAY_MS = 1000
+const RECONNECT_MAX_DELAY_MS = 30_000
+
+export class RealtimeConnection {
+  private channel: RealtimeChannel | null = null
+  private heartbeatTimer: ReturnType<typeof setInterval> | null = null
+  private reconnectTimer: ReturnType<typeof setTimeout> | null = null
+  private reconnectAttempts = 0
+  private projectId: string
+  private userId: string
+  private callbacks: RealtimeCallbacks
+  private isDestroyed = false
+  private supabase = createClient()
+
+  constructor(projectId: string, userId: string, callbacks: RealtimeCallbacks) {
+    this.projectId = projectId
+    this.userId = userId
+    this.callbacks = callbacks
+  }
+
+  async connect(): Promise<void> {
+    if (this.isDestroyed) return
+    this.callbacks.onConnectionStateChange('connecting')
+
+    this.channel = this.supabase.channel(`project:${this.projectId}`, {
+      config: { presence: { key: this.userId } },
+    })
+
+    this.channel
+      .on('presence', { event: 'sync' }, () => {
+        if (this.channel) {
+          const state = this.channel.presenceState()
+          this.callbacks.onPresenceSync?.(state)
+        }
+      })
+      .subscribe(async (status) => {
+        if (this.isDestroyed) return
+
+        if (status === 'SUBSCRIBED') {
+          this.reconnectAttempts = 0
+          this.callbacks.onConnectionStateChange('connected')
+          this.startHeartbeat()
+          await this.createSession()
+        } else if (status === 'CHANNEL_ERROR' || status === 'TIMED_OUT') {
+          this.callbacks.onConnectionStateChange('reconnecting')
+          this.scheduleReconnect()
+        } else if (status === 'CLOSED') {
+          this.callbacks.onConnectionStateChange('disconnected')
+        }
+      })
+  }
+
+  async disconnect(): Promise<void> {
+    this.isDestroyed = true
+    this.stopHeartbeat()
+    this.clearReconnectTimer()
+
+    if (this.channel) {
+      await this.deleteSession()
+      this.supabase.removeChannel(this.channel)
+      this.channel = null
+    }
+
+    this.callbacks.onConnectionStateChange('disconnected')
+  }
+
+  getChannel(): RealtimeChannel | null {
+    return this.channel
+  }
+
+  private async createSession(): Promise<void> {
+    try {
+      await this.supabase.from('collaboration_sessions').upsert(
+        {
+          project_id: this.projectId,
+          user_id: this.userId,
+          cursor_position: null,
+          selected_node_id: null,
+          connected_at: new Date().toISOString(),
+          last_heartbeat: new Date().toISOString(),
+        },
+        { onConflict: 'project_id,user_id' }
+      )
+    } catch {
+      // Fire-and-forget: session creation failure shouldn't block editing
+    }
+  }
+
+  private async deleteSession(): Promise<void> {
+    try {
+      await this.supabase
+        .from('collaboration_sessions')
+        .delete()
+        .eq('project_id', this.projectId)
+        .eq('user_id', this.userId)
+    } catch {
+      // Fire-and-forget: cleanup failure is non-critical
+    }
+  }
+
+  private startHeartbeat(): void {
+    this.stopHeartbeat()
+    this.heartbeatTimer = setInterval(async () => {
+      if (this.isDestroyed) {
+        this.stopHeartbeat()
+        return
+      }
+      try {
+        await this.supabase
+          .from('collaboration_sessions')
+          .update({ last_heartbeat: new Date().toISOString() })
+          .eq('project_id', this.projectId)
+          .eq('user_id', this.userId)
+      } catch {
+        // Heartbeat failure is non-critical
+      }
+    }, HEARTBEAT_INTERVAL_MS)
+  }
+
+  private stopHeartbeat(): void {
+    if (this.heartbeatTimer) {
+      clearInterval(this.heartbeatTimer)
+      this.heartbeatTimer = null
+    }
+  }
+
+  private scheduleReconnect(): void {
+    if (this.isDestroyed) return
+    this.clearReconnectTimer()
+
+    const delay = Math.min(
+      RECONNECT_BASE_DELAY_MS * Math.pow(2, this.reconnectAttempts),
+      RECONNECT_MAX_DELAY_MS
+    )
+    this.reconnectAttempts++
+
+    this.reconnectTimer = setTimeout(async () => {
+      if (this.isDestroyed) return
+      this.callbacks.onConnectionStateChange('reconnecting')
+
+      if (this.channel) {
+        this.supabase.removeChannel(this.channel)
+        this.channel = null
+      }
+
+      await this.connect()
+    }, delay)
+  }
+
+  private clearReconnectTimer(): void {
+    if (this.reconnectTimer) {
+      clearTimeout(this.reconnectTimer)
+      this.reconnectTimer = null
+    }
+  }
+}
diff --git a/supabase/migrations/20260123200000_add_collaboration_sessions_unique_constraint.sql b/supabase/migrations/20260123200000_add_collaboration_sessions_unique_constraint.sql
new file mode 100644
index 0000000..a54d00a
--- /dev/null
+++ b/supabase/migrations/20260123200000_add_collaboration_sessions_unique_constraint.sql
@@ -0,0 +1,6 @@
+-- Add unique constraint on collaboration_sessions for (project_id, user_id)
+-- This ensures a user can only have one active session per project,
+-- and allows upsert operations for session management.
+ALTER TABLE collaboration_sessions
+  ADD CONSTRAINT collaboration_sessions_project_user_unique
+  UNIQUE (project_id, user_id);