diff --git a/ddtrace/llmobs/_llmobs.py b/ddtrace/llmobs/_llmobs.py
index 21d8174dc6d..a24fd4b355b 100644
--- a/ddtrace/llmobs/_llmobs.py
+++ b/ddtrace/llmobs/_llmobs.py
@@ -1762,6 +1762,11 @@ def submit_evaluation(
                             "Failed to parse tags. Tags for evaluation metrics must be strings."
                         )
 
+            # Auto-add source:otel tag when OTel tracing is enabled
+            # This allows the backend to wait for OTel span conversion
+            if config._otel_trace_enabled:
+                evaluation_tags["source"] = "otel"
+
             evaluation_metric: LLMObsEvaluationMetricEvent = {
                 "join_on": join_on,
                 "label": str(label),
diff --git a/tests/llmobs/test_llmobs_otel_evaluation.py b/tests/llmobs/test_llmobs_otel_evaluation.py
new file mode 100644
index 00000000000..da34a46b368
--- /dev/null
+++ b/tests/llmobs/test_llmobs_otel_evaluation.py
@@ -0,0 +1,45 @@
+"""
+Tests for automatic source:otel tag on evaluations when OTel tracing is enabled.
+
+When DD_TRACE_OTEL_ENABLED=true, all evaluations should have `source:otel` tag
+to allow the backend to wait for OTel span conversion (~3 minutes) before
+discarding unmatched evaluations.
+"""
+
+import mock
+
+
+def test_submit_evaluation_adds_source_otel_when_otel_enabled(llmobs, mock_llmobs_eval_metric_writer):
+    """Verify source:otel tag is auto-added when DD_TRACE_OTEL_ENABLED=true."""
+    with mock.patch("ddtrace.llmobs._llmobs.config._otel_trace_enabled", True):
+        llmobs.submit_evaluation(
+            span={"span_id": "123", "trace_id": "456"},
+            label="quality",
+            metric_type="score",
+            value=0.9,
+            ml_app="test-app",
+        )
+
+    mock_llmobs_eval_metric_writer.enqueue.assert_called_once()
+    call_args = mock_llmobs_eval_metric_writer.enqueue.call_args[0][0]
+
+    assert "tags" in call_args
+    assert "source:otel" in call_args["tags"]
+
+
+def test_submit_evaluation_no_source_otel_when_otel_disabled(llmobs, mock_llmobs_eval_metric_writer):
+    """Verify source:otel tag is NOT added when DD_TRACE_OTEL_ENABLED=false (default)."""
+    with mock.patch("ddtrace.llmobs._llmobs.config._otel_trace_enabled", False):
+        llmobs.submit_evaluation(
+            span={"span_id": "123", "trace_id": "456"},
+            label="quality",
+            metric_type="score",
+            value=0.9,
+            ml_app="test-app",
+        )
+
+    mock_llmobs_eval_metric_writer.enqueue.assert_called_once()
+    call_args = mock_llmobs_eval_metric_writer.enqueue.call_args[0][0]
+
+    assert "tags" in call_args
+    assert "source:otel" not in call_args["tags"]