diff --git a/docs/STELLAR_MEMO_GROUP_ID.md b/docs/STELLAR_MEMO_GROUP_ID.md
new file mode 100644
index 0000000..94d4e04
--- /dev/null
+++ b/docs/STELLAR_MEMO_GROUP_ID.md
@@ -0,0 +1,253 @@
+# Stellar Memo Group ID Feature
+
+## Overview
+
+This feature leverages Stellar's native memo field to store and reference group IDs during on-chain interactions. This enables lightweight group identification without requiring additional custom fields or contracts.
+
+## Architecture
+
+### Memo Format
+
+- **Format**: `grp_<12-char-slug>` (17 bytes total, well within Stellar's 28-byte limit)
+- **Derivation**: Deterministic derivation from room ID using the existing format `room_{timestamp}_{random9chars}`
+- **Fallback**: SHA-256 hash of room ID (first 24 hex chars) for non-standard room IDs
+
+### Database Schema
+
+#### `rooms` table
+- **`memo_group_id`**: Compact group identifier (≤28 bytes) embedded in the Stellar transaction memo field
+- **Index**: Unique index on `memo_group_id` for fast lookups
+
+#### `group_tx_memos` table
+Lookup table for groupId ↔ transactionId mapping:
+- **`group_id`**: References `rooms(id)` with CASCADE delete
+- **`memo_group_id`**: The exact text stored in the Stellar transaction memo
+- **`tx_hash`**: Stellar transaction hash that contains this memo
+- **Indexes**: Fast lookups in both directions (group_id, memo_group_id, tx_hash)
+- **RLS**: Public read access, authenticated insert only
+
+## Implementation Details
+
+### Core Components
+
+#### 1. Memo Utilities (`lib/blockchain/memo.ts`)
+
+```typescript
+// Derive a deterministic memo from room ID
+deriveMemoGroupId(roomId: string): string
+
+// Validate memo format and constraints
+validateMemoGroupId(memo: unknown): { valid: true } | { valid: false; reason: string }
+
+// Check if memo matches expected value for a group
+memoMatchesGroup(roomId: string, onChainMemo: string): boolean
+```
+
+**Validation Rules**:
+- Must be a non-empty string
+- Must be ≤28 bytes when encoded as UTF-8
+- Must start with "grp_" prefix
+- Must contain only printable ASCII characters
+
+#### 2. Memo Middleware (`lib/blockchain/memo-middleware.ts`)
+
+```typescript
+// Validate memo from request
+validateRequestMemo(memo: unknown, groupId?: string): MemoValidationResult
+
+// Return error response for invalid memo
+memoValidationError(memo: unknown, groupId?: string): NextResponse | null
+```
+
+#### 3. Stellar Service Integration (`lib/blockchain/stellar-service.ts`)
+
+The `submitMetadataHash()` function:
+1. Derives memo from group ID using `deriveMemoGroupId()`
+2. Validates memo before building transaction
+3. Embeds memo in Stellar transaction using `StellarSdk.Memo.text()`
+4. Returns `memoGroupId` in the result for database storage
+
+#### 4. API Endpoints
+
+##### `GET /api/stellar/memo?memo=grp_abc123xyz`
+Looks up a group by its Stellar memo identifier:
+- Validates memo format
+- Queries `group_tx_memos` lookup table first (fast path)
+- Falls back to `rooms` table if lookup table entry missing
+- Returns room record and associated transaction hash
+
+##### `POST /api/rooms`
+Creates a room and:
+- Derives memo from room ID
+- Submits transaction with embedded memo
+- Stores `memo_group_id` in `rooms` table
+- Persists mapping in `group_tx_memos` lookup table
+
+##### `GET /api/rooms/[roomId]/verify`
+Verifies room metadata including:
+- Retrieves transaction from blockchain
+- Extracts memo from on-chain transaction
+- Validates memo matches expected value for group
+- Returns verification status with memo validation result
+
+## Acceptance Criteria
+
+✅ **Each transaction memo is linked to a specific group ID**
+- Memo is deterministically derived from room ID
+- Stored in both `rooms.memo_group_id` and `group_tx_memos` table
+
+✅ **Memo is stored and retrievable during on-chain interaction**
+- Memo embedded in Stellar transaction via `StellarSdk.Memo.text()`
+- Can be retrieved via `/api/stellar/memo` endpoint
+- Transaction hash stored for blockchain verification
+
+✅ **Group ID can be validated against existing records**
+- `memoMatchesGroup()` validates on-chain memo against expected value
+- Middleware validates memo format and cross-checks with group ID
+- Verification endpoint includes memo validation status
+
+✅ **Ensure memo usage does not conflict with other transaction metadata**
+- Memo carries group ID reference only
+- Metadata hash stored separately in database
+- Clear separation of concerns: memo = group ID, hash = metadata integrity
+
+## Migration
+
+Apply the database migration:
+
+```bash
+# Using psql
+psql "$DATABASE_URL" -f scripts/011_stellar_memo_group_id.sql
+
+# Or via Supabase SQL Editor
+# Run the contents of scripts/011_stellar_memo_group_id.sql
+```
+
+## Usage Examples
+
+### Deriving a Memo
+
+```typescript
+import { deriveMemoGroupId } from "@/lib/blockchain/memo";
+
+const roomId = "room_1714000000000_abc123xyz";
+const memo = deriveMemoGroupId(roomId);
+// Returns: "grp_abc123xyz" (17 bytes)
+```
+
+### Validating a Memo
+
+```typescript
+import { validateMemoGroupId } from "@/lib/blockchain/memo";
+
+const result = validateMemoGroupId("grp_abc123xyz");
+if (!result.valid) {
+  console.error(result.reason);
+}
+```
+
+### Looking Up a Group by Memo
+
+```bash
+curl "https://your-app.com/api/stellar/memo?memo=grp_abc123xyz"
+```
+
+Response:
+```json
+{
+  "groupId": "room_1714000000000_abc123xyz",
+  "memoGroupId": "grp_abc123xyz",
+  "transactionHash": "abc123...",
+  "explorerUrl": "https://stellar.expert/tx/abc123...",
+  "room": {
+    "id": "room_1714000000000_abc123xyz",
+    "name": "My Group",
+    "description": "A private group",
+    "is_private": false,
+    "created_at": "2024-01-01T00:00:00Z"
+  }
+}
+```
+
+### Verifying Memo Integrity
+
+```typescript
+import { memoMatchesGroup } from "@/lib/blockchain/memo";
+
+const roomId = "room_1714000000000_abc123xyz";
+const onChainMemo = "grp_abc123xyz";
+
+if (memoMatchesGroup(roomId, onChainMemo)) {
+  console.log("Memo is valid for this group");
+}
+```
+
+## Error Handling
+
+### Missing Memo
+- API returns 400 with error: "memo query parameter is required"
+
+### Invalid Memo Format
+- API returns 400 with detailed reason (e.g., exceeds byte limit, wrong prefix, non-ASCII characters)
+
+### Memo Not Found
+- API returns 404 with error: "No group found for the provided memo"
+
+### Memo Mismatch
+- Verification endpoint returns `memoVerified: false` with expected vs actual memo values
+
+## Technical Notes
+
+### Memo Length Limits
+- Stellar TEXT memo limit: 28 bytes
+- Our format: `grp_<12-char-slug>` = 17 bytes (safe margin)
+- Fallback format: `grp_<24-hex-chars>` = 28 bytes (at limit)
+
+### Deterministic Derivation
+- Same room ID always produces same memo
+- Enables reliable on-chain group identification
+- No need to store memo separately for derivation
+
+### Lookup Table Benefits
+- Fast O(1) lookups by memo or transaction hash
+- Avoids re-querying blockchain for common operations
+- Enables reverse lookup (tx_hash → group_id)
+
+### Security Considerations
+- Memo is public on-chain (by design)
+- Group IDs are pseudonymous, not sensitive
+- RLS policies ensure authenticated inserts only
+- Memo validation prevents injection attacks
+
+## Testing
+
+### Manual Testing
+
+1. Create a room via POST `/api/rooms`
+2. Verify `memo_group_id` is returned in response
+3. Check database for `memo_group_id` in `rooms` table
+4. Check database for entry in `group_tx_memos` table
+5. Lookup group via GET `/api/stellar/memo?memo=grp_xxx`
+6. Verify room via GET `/api/rooms/[roomId]/verify`
+7. Confirm `memoVerified: true` in verification response
+
+### API Smoke Test
+
+```bash
+# Test memo lookup with valid memo
+curl "http://localhost:3000/api/stellar/memo?memo=grp_test123"
+
+# Test memo lookup with invalid memo
+curl "http://localhost:3000/api/stellar/memo?memo=invalid"
+
+# Test memo lookup with missing parameter
+curl "http://localhost:3000/api/stellar/memo"
+```
+
+## Future Enhancements
+
+- [ ] Add memo-based group search in frontend
+- [ ] Implement memo-based group discovery
+- [ ] Add memo validation to all group-related endpoints
+- [ ] Consider using HASH memo for additional privacy
+- [ ] Add memo rotation support for group renames
diff --git a/scripts/MIGRATIONS_README.md b/scripts/MIGRATIONS_README.md
index 72de07c..79fe5c5 100644
--- a/scripts/MIGRATIONS_README.md
+++ b/scripts/MIGRATIONS_README.md
@@ -12,6 +12,7 @@ Included migrations (apply in numeric order):
 - `006_unread_view.sql`          (new)
 - `007_create_group_membership.sql`  (new)
 - `011_enhance_invites_for_group_join.sql` (new)
+- `011_stellar_memo_group_id.sql` (new)
 
 ## How to apply (psql)
 
@@ -30,6 +31,7 @@ psql "$DATABASE_URL" -f scripts/005_add_last_read_to_room_members.sql
 psql "$DATABASE_URL" -f scripts/006_unread_view.sql
 psql "$DATABASE_URL" -f scripts/007_create_group_membership.sql
 psql "$DATABASE_URL" -f scripts/011_enhance_invites_for_group_join.sql
+psql "$DATABASE_URL" -f scripts/011_stellar_memo_group_id.sql
 ```
 
 ## How to apply (Supabase)
@@ -43,6 +45,7 @@ If the project uses Supabase, maintainers can run the same `psql` commands again
 - `scripts/006_unread_view.sql` creates `public.user_room_unreads` view and grants `SELECT` to `public` for convenience; adjust privileges as needed.
 - `scripts/007_create_group_membership.sql` creates `public.group_membership` table for wallet-based group membership tracking.
 - `scripts/011_enhance_invites_for_group_join.sql` adds `max_uses` and `use_count` columns to `public.invites`, adds RLS policies for authenticated read/update, and creates the `increment_invite_use_count` SQL function for atomic usage tracking.
+- `scripts/011_stellar_memo_group_id.sql` adds `memo_group_id` column to `public.rooms` and creates `public.group_tx_memos` lookup table for groupId ↔ transactionId mapping via Stellar memo field. This enables lightweight group identification from on-chain transactions.
 - A development-only endpoint (`/api/rooms/seed-test`) was added that seeds a room for an authenticated user. It requires a valid Supabase session; do not enable any service-role or unauthenticated behavior in production without review.
 
 ## Including migrations in PRs