diff --git a/onnxruntime/core/graph/graph.cc b/onnxruntime/core/graph/graph.cc
index 38b2df4c78..44b283b8ce 100644
--- a/onnxruntime/core/graph/graph.cc
+++ b/onnxruntime/core/graph/graph.cc
@@ -1305,18 +1305,32 @@ Status Graph::InferAndVerifySubgraphTypes(const Node& node, Graph& subgraph,
 
   output_types.clear();
 
-  auto& subgraph_inputs = subgraph.GetInputs();
-  auto num_subgraph_inputs = subgraph_inputs.size();
+  // the spec says all inputs should be provided for the subgraph so default to that first
+  auto* subgraph_inputs = &subgraph.GetInputsIncludingInitializers();
+  auto num_subgraph_inputs = subgraph_inputs->size();
 
   if (num_subgraph_inputs != input_types.size()) {
-    return ORT_MAKE_STATUS(ONNXRUNTIME, FAIL, "Size mismatch validating subgraph inputs. Got ",
-                           input_types.size(), " inputs but subgraph requires ", subgraph_inputs.size());
+    // we also allow for just the required inputs to be provided to be user friendly due to ONNX requiring
+    // initializers to have matching inputs (making them optional inputs that most likely the user doesn't want to
+    // override).
+    auto& required_subgraph_inputs = subgraph.GetInputs();
+    auto num_required_subgraph_inputs = required_subgraph_inputs.size();
+
+    if (num_required_subgraph_inputs != input_types.size()) {
+      return ORT_MAKE_STATUS(ONNXRUNTIME, FAIL, "Size mismatch validating subgraph inputs. Got ", input_types.size(),
+                             " inputs but subgraph has ", num_subgraph_inputs,
+                             " inputs and requires ", num_required_subgraph_inputs,
+                             " inputs. Either provide all subgraph inputs, or just the required inputs.");
+    } else {
+      subgraph_inputs = &required_subgraph_inputs;
+      num_subgraph_inputs = num_required_subgraph_inputs;
+    }
   }
 
   // apply type/shape info to the subgraph's inputs
   for (size_t i = 0; i < num_subgraph_inputs; ++i) {
     const auto& input_type = *input_types[i];
-    const auto& subgraph_input = *subgraph_inputs[i];
+    const auto& subgraph_input = *subgraph_inputs->at(i);
 
     NodeArg* mutable_nodearg = subgraph.GetNodeArg(subgraph_input.Name());
     status = mutable_nodearg->UpdateTypeAndShape(input_type);
diff --git a/onnxruntime/core/providers/cpu/controlflow/scan_utils.cc b/onnxruntime/core/providers/cpu/controlflow/scan_utils.cc
index 444c471b42..d341314006 100644
--- a/onnxruntime/core/providers/cpu/controlflow/scan_utils.cc
+++ b/onnxruntime/core/providers/cpu/controlflow/scan_utils.cc
@@ -96,10 +96,19 @@ Status IterateSequence(OpKernelContextInternal& context,
                        std::vector<std::string>& subgraph_output_names,
                        std::vector<std::unique_ptr<OutputIterator>>& output_iterators) {
   Status status = Status::OK();
-  auto& graph_inputs = subgraph.GetInputsIncludingInitializers();
+
+  // prefer matching all inputs to the subgraph as per the Scan spec,
+  auto* graph_inputs = &subgraph.GetInputsIncludingInitializers();
+  if (static_cast<size_t>(num_variadic_inputs) < graph_inputs->size()) {
+    // fallback to just the required inputs.
+    graph_inputs = &subgraph.GetInputs();
+    ORT_ENFORCE(static_cast<size_t>(num_variadic_inputs) == graph_inputs->size(),
+                "Graph::InferAndVerifySubgraphTypes should have already validated that "
+                "num_variadic_inputs matched the subgraph inputs or required inputs.");
+  }
+
   NameMLValMap feeds;
   std::vector<MLValue> fetches;
-
   feeds.reserve(num_variadic_inputs + implicit_inputs.size());
   fetches.resize(num_variadic_outputs);
 
@@ -113,7 +122,7 @@ Status IterateSequence(OpKernelContextInternal& context,
   for (; seq_no < seq_length; ++seq_no) {
     for (int input = 0; input < num_variadic_inputs; ++input) {
       // the ordering of the Scan inputs should match the ordering of the subgraph inputs
-      auto name = graph_inputs[input]->Name();
+      auto name = (*graph_inputs)[input]->Name();
 
       if (input < num_loop_state_variables) {
         // add loop state variable input
diff --git a/onnxruntime/test/providers/cpu/controlflow/loop_test.cc b/onnxruntime/test/providers/cpu/controlflow/loop_test.cc
index ad13abc936..98e2ddf98b 100644
--- a/onnxruntime/test/providers/cpu/controlflow/loop_test.cc
+++ b/onnxruntime/test/providers/cpu/controlflow/loop_test.cc
@@ -267,6 +267,18 @@ static const ONNX_NAMESPACE::GraphProto CreateSubgraph(const RunOptions& options
   graph.SetInputOrder({&iter_num_in, &cond_in, &loop_var_0_in, &loop_var_1_in});
   graph.SetOutputOrder({cond_out, loop_var_0_out, loop_var_1_out, loop_out_0});
 
+  // optional input backed by an initializer to make sure that's handled too.
+  // we expect that Graph::InferAndVerifySubgraphTypes will be able to ignore the optional input if not provided
+  {
+    TensorProto optional_input_tensor;
+    optional_input_tensor.set_name("optional_float");
+    optional_input_tensor.add_dims(1);
+    optional_input_tensor.add_float_data(1.f);
+    optional_input_tensor.set_data_type(onnx::TensorProto_DataType_FLOAT);
+
+    graph.AddInitializedTensor(optional_input_tensor);
+  }
+
   auto status = graph.Resolve();
   EXPECT_EQ(status, Status::OK());