diff --git a/python/tvm/relax/frontend/onnx/onnx_frontend.py b/python/tvm/relax/frontend/onnx/onnx_frontend.py
index 3dc575ae778c..1aa7b36e801f 100644
--- a/python/tvm/relax/frontend/onnx/onnx_frontend.py
+++ b/python/tvm/relax/frontend/onnx/onnx_frontend.py
@@ -302,8 +302,13 @@ def get_converter(cls, opset):
             number smaller than or equal to opset belongs to all support versions.
         """
         versions = [int(d.replace("_impl_v", "")) for d in dir(cls) if "_impl_v" in d]
-        versions = sorted(versions + [opset])
-        version = versions[max([i for i, v in enumerate(versions) if v == opset]) - 1]
+        compatible = [v for v in versions if v <= opset]
+        if not compatible:
+            raise NotImplementedError(
+                f"{cls.__name__} is not supported for opset {opset}. "
+                f"Minimum supported opset: {min(versions)}"
+            )
+        version = max(compatible)
         if hasattr(cls, f"_impl_v{version}"):
             return getattr(cls, f"_impl_v{version}")
         raise NotImplementedError(f"opset version {version} of {cls.__name__} not implemented")
diff --git a/tests/python/relax/test_frontend_onnx.py b/tests/python/relax/test_frontend_onnx.py
index ecbc6c9e8a5e..475659852989 100644
--- a/tests/python/relax/test_frontend_onnx.py
+++ b/tests/python/relax/test_frontend_onnx.py
@@ -3980,5 +3980,35 @@ def test_nms_score_threshold():
         )
 
 
+def test_reduce_mean():
+    # opset 13: axes passed as attribute
+    node = helper.make_node("ReduceMean", inputs=["x"], outputs=["y"], axes=[2], keepdims=1)
+    graph = helper.make_graph(
+        [node],
+        "reduce_mean_test",
+        inputs=[helper.make_tensor_value_info("x", TensorProto.FLOAT, [1, 68, 4, 18])],
+        outputs=[helper.make_tensor_value_info("y", TensorProto.FLOAT, [1, 68, 1, 18])],
+    )
+    model = helper.make_model(graph, producer_name="reduce_mean_test")
+    check_correctness(model, opset=13)
+
+
+def test_reduce_mean_unsupported_opset():
+    # Regression test for https://github.com/apache/tvm/issues/18698.
+    # When opset < minimum available impl version, get_converter previously
+    # wrapped to -1 and silently picked the newest impl instead of raising.
+    node = helper.make_node("ReduceMean", inputs=["x"], outputs=["y"], axes=[2], keepdims=1)
+    graph = helper.make_graph(
+        [node],
+        "reduce_mean_test",
+        inputs=[helper.make_tensor_value_info("x", TensorProto.FLOAT, [1, 68, 4, 18])],
+        outputs=[helper.make_tensor_value_info("y", TensorProto.FLOAT, [1, 68, 1, 18])],
+    )
+    model = helper.make_model(graph, producer_name="reduce_mean_test")
+    model.opset_import[0].version = 9
+    with pytest.raises(NotImplementedError, match="not supported for opset 9"):
+        from_onnx(model, opset=9, keep_params_in_input=True)
+
+
 if __name__ == "__main__":
     tvm.testing.main()