diff --git a/backend/main.py b/backend/main.py
index a434e29..81f49f6 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -1,3 +1,5 @@
+from datetime import UTC, datetime
+
 from fastapi import FastAPI, HTTPException, Query, Request, Response
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
@@ -272,14 +274,33 @@ def get_rmas():
     return db_get("rmas", "order=created_at.desc")
 
 
+# Resolution statuses that end an RMA's lifecycle. Moving into one of these
+# stamps rmas.closed_at; moving back out (reopening) clears it.
+RMA_CLOSED_STATUSES = {"Completed", "Cancelled"}
+
+
+def utc_now_iso() -> str:
+    return datetime.now(UTC).isoformat()
+
+
 @app.post("/rmas")
 def create_rma(r: RMA):
-    return db_post("rmas", model_data(r))
+    data = model_data(r)
+    data["closed_at"] = utc_now_iso() if data["resolution_status"] in RMA_CLOSED_STATUSES else None
+    return db_post("rmas", data)
 
 
 @app.put("/rmas/{id}")
 def update_rma(id: str, r: RMA):
-    return db_patch("rmas", id, model_data(r))
+    data = model_data(r)
+    if data["resolution_status"] in RMA_CLOSED_STATUSES:
+        # Keep the original close time when an already-closed RMA is edited again.
+        old = db_get("rmas", f"id=eq.{escape_filter_value(id)}&select=closed_at")
+        if not (old and old[0].get("closed_at")):
+            data["closed_at"] = utc_now_iso()
+    else:
+        data["closed_at"] = None
+    return db_patch("rmas", id, data)
 
 
 # --- Dashboard ---
diff --git a/backend/test_main.py b/backend/test_main.py
index ebd5e80..d12ea09 100644
--- a/backend/test_main.py
+++ b/backend/test_main.py
@@ -5,6 +5,7 @@
 
 import sys
 import types
+from datetime import datetime
 from unittest.mock import MagicMock
 from urllib.parse import quote, unquote
 
@@ -507,6 +508,66 @@ def test_rma_routes_delegate_to_database(client, monkeypatch):
     assert calls[1][0:3] == ("patch", "rmas", "r1")
 
 
+def _capture_rma_writes(main, monkeypatch, existing_closed_at=None):
+    calls = []
+    monkeypatch.setattr(
+        main, "db_get", lambda table, params="": [{"closed_at": existing_closed_at}]
+    )
+    monkeypatch.setattr(main, "db_post", lambda table, data: calls.append(data) or data)
+    monkeypatch.setattr(main, "db_patch", lambda table, row_id, data: calls.append(data) or data)
+    return calls
+
+
+def test_create_rma_sets_closed_at_only_for_closed_status(client, monkeypatch):
+    test_client, main = client
+    calls = _capture_rma_writes(main, monkeypatch)
+
+    test_client.post("/rmas", json={"ticket_id": "t1", "rma_number": "RMA-1"})
+    test_client.post(
+        "/rmas",
+        json={"ticket_id": "t1", "rma_number": "RMA-2", "resolution_status": "Completed"},
+    )
+
+    assert calls[0]["closed_at"] is None
+    assert datetime.fromisoformat(calls[1]["closed_at"]).tzinfo is not None
+
+
+def test_update_rma_stamps_closed_at_when_closing(client, monkeypatch):
+    test_client, main = client
+    calls = _capture_rma_writes(main, monkeypatch, existing_closed_at=None)
+
+    test_client.put(
+        "/rmas/r1",
+        json={"ticket_id": "t1", "rma_number": "RMA-1", "resolution_status": "Cancelled"},
+    )
+
+    assert datetime.fromisoformat(calls[0]["closed_at"]).tzinfo is not None
+
+
+def test_update_rma_keeps_existing_closed_at(client, monkeypatch):
+    test_client, main = client
+    calls = _capture_rma_writes(main, monkeypatch, existing_closed_at="2026-01-01T00:00:00+00:00")
+
+    test_client.put(
+        "/rmas/r1",
+        json={"ticket_id": "t1", "rma_number": "RMA-1", "resolution_status": "Completed"},
+    )
+
+    assert "closed_at" not in calls[0]
+
+
+def test_update_rma_clears_closed_at_when_reopened(client, monkeypatch):
+    test_client, main = client
+    calls = _capture_rma_writes(main, monkeypatch, existing_closed_at="2026-01-01T00:00:00+00:00")
+
+    test_client.put(
+        "/rmas/r1",
+        json={"ticket_id": "t1", "rma_number": "RMA-1", "resolution_status": "In Progress"},
+    )
+
+    assert calls[0]["closed_at"] is None
+
+
 def test_dashboard_counts_ticket_and_rma_statuses(client, monkeypatch):
     test_client, main = client