diff --git a/backend/services/negotiation_service.py b/backend/services/negotiation_service.py
index 10a11f6..f551bf2 100644
--- a/backend/services/negotiation_service.py
+++ b/backend/services/negotiation_service.py
@@ -112,6 +112,12 @@ class NegotiationService:
         )
         if err_type != ErrorType.SUCCESS or quote is None:
             return ErrorType.NEGO_NOT_FOUND, None, None
+
+        # 협상완료 세션은 결과 열람용 재진입(무변경)이므로 견적마감·마감시간 검증을 건너뛴다.
+        # reject 는 blocked_statuses 로 DONE 을 이미 막으므로 이 분기는 participate 에만 닿는다.
+        if sess.status == SessionStatus.DONE.value:
+            return ErrorType.SUCCESS, sess, quote
+
         if quote.status == QuotationStatus.CLOSED.value:
             return ErrorType.NEGO_QUOTATION_CLOSED, None, None
 
diff --git a/backend/tests/test_negotiation.py b/backend/tests/test_negotiation.py
index 8c68603..4c5663c 100644
--- a/backend/tests/test_negotiation.py
+++ b/backend/tests/test_negotiation.py
@@ -221,6 +221,22 @@ async def test_participate_in_progress_no_state_change(client, nego_seed, db_eng
     assert await _quotation_status(db_engine, qid) == 2    # 무변경
 
 
+async def test_participate_done_bypasses_quotation_closed(client, nego_seed, db_engine):
+    """협상완료 세션은 결과 열람용 재진입이므로 견적마감·마감시간이 지나도 참여(진입) 가능."""
+    token = await _login_token(client)
+    sid, qid = nego_seed["sids"]["C"], nego_seed["qids"]["C"]  # 협상완료
+    async with db_engine.begin() as conn:
+        await conn.execute(
+            text("UPDATE quotation.quotations SET status = 3, end_time = now() - make_interval(hours => 1) WHERE qt_id = :qid"),
+            {"qid": qid},
+        )  # 견적마감 + 마감시간 경과
+    r = await _participate(client, token, sid)
+    assert r.json()["result"]["success"] is True
+    assert r.json()["session_id"] == str(sid)
+    assert await _session_status(db_engine, sid) == 3      # 무변경 (협상완료 유지)
+    assert await _quotation_status(db_engine, qid) == 3    # 무변경 (견적마감 유지)
+
+
 async def test_participate_session_not_found(client, nego_seed):
     token = await _login_token(client)
     r = await _participate(client, token, str(uuid.uuid4()))