diff --git a/src/reader/spirv/function.cc b/src/reader/spirv/function.cc
index 2024a2fb26..a0de6f829a 100644
--- a/src/reader/spirv/function.cc
+++ b/src/reader/spirv/function.cc
@@ -20,6 +20,8 @@
 #include "src/ast/assignment_statement.h"
 #include "src/ast/bitcast_expression.h"
 #include "src/ast/break_statement.h"
+#include "src/ast/builtin.h"
+#include "src/ast/builtin_decoration.h"
 #include "src/ast/call_statement.h"
 #include "src/ast/continue_statement.h"
 #include "src/ast/discard_statement.h"
@@ -1026,24 +1028,37 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
     const auto return_struct_sym =
         builder_.Symbols().Register(return_struct_name);
 
+    const auto& builtin_position_info = parser_impl_.GetBuiltInPositionInfo();
+
     // Define the structure.
     ast::ExpressionList return_exprs;
     std::vector<ast::StructMember*> return_members;
     for (uint32_t var_id : ep_info_->outputs) {
-      const auto* var = def_use_mgr_->GetDef(var_id);
-      TINT_ASSERT(var != nullptr);
-      TINT_ASSERT(var->opcode() == SpvOpVariable);
-      const auto* store_type = GetVariableStoreType(*var);
-      const auto* forced_store_type = store_type;
+      const Type* param_type = nullptr;
+      const Type* store_type = nullptr;
       ast::DecorationList out_decos;
-      if (!parser_impl_.ConvertDecorationsForVariable(
-              var_id, &forced_store_type, &out_decos)) {
-        // This occurs, and is not an error, for the PointSize builtin.
-        if (!success()) {
-          // But exit early if an error was logged.
-          return false;
+      if (var_id == builtin_position_info.per_vertex_var_id) {
+        // The SPIR-V gl_PerVertex variable has already been remapped to
+        // a gl_Position variable.
+        // Substitute the type.
+        param_type = ty_.Vector(ty_.F32(), 4);
+        out_decos.emplace_back(
+            create<ast::BuiltinDecoration>(source, ast::Builtin::kPosition));
+      } else {
+        const auto* var = def_use_mgr_->GetDef(var_id);
+        TINT_ASSERT(var != nullptr);
+        TINT_ASSERT(var->opcode() == SpvOpVariable);
+        store_type = GetVariableStoreType(*var);
+        param_type = store_type;
+        if (!parser_impl_.ConvertDecorationsForVariable(var_id, &param_type,
+                                                        &out_decos)) {
+          // This occurs, and is not an error, for the PointSize builtin.
+          if (!success()) {
+            // But exit early if an error was logged.
+            return false;
+          }
+          continue;
         }
-        continue;
       }
 
       // TODO(dneto): flatten structs and arrays to vectors or scalars.
@@ -1057,7 +1072,7 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
       // Form the member type.
       // Reuse the var name for the member name. They can't clash.
       ast::StructMember* return_member = create<ast::StructMember>(
-          Source{}, var_sym, forced_store_type->Build(builder_), out_decos);
+          Source{}, var_sym, param_type->Build(builder_), out_decos);
       return_members.push_back(return_member);
 
       ast::Expression* return_member_value =
@@ -1070,7 +1085,7 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
         if (store_type->As<Array>()->type->IsSignedScalarOrVector()) {
           // sample_mask is unsigned in WGSL. Bitcast it.
           return_member_value = create<ast::BitcastExpression>(
-              source, forced_store_type->Build(builder_), return_member_value);
+              source, param_type->Build(builder_), return_member_value);
         }
       } else {
         // No other builtin outputs need signedness conversion.
diff --git a/src/reader/spirv/parser_impl.cc b/src/reader/spirv/parser_impl.cc
index 01df33cd53..04e2e5d363 100644
--- a/src/reader/spirv/parser_impl.cc
+++ b/src/reader/spirv/parser_impl.cc
@@ -1053,7 +1053,8 @@ const Type* ParserImpl::ConvertType(uint32_t type_id,
 
   if (pointee_type_id == builtin_position_.struct_type_id) {
     builtin_position_.pointer_type_id = type_id;
-    builtin_position_.storage_class = storage_class;
+    builtin_position_.storage_class =
+        hlsl_style_pipeline_io_ ? SpvStorageClassPrivate : storage_class;
     return nullptr;
   }
   auto* ast_elem_ty = ConvertType(pointee_type_id, PtrAs::Ptr);
@@ -1314,13 +1315,18 @@ bool ParserImpl::EmitModuleScopeVariables() {
     // Make sure the variable has a name.
     namer_.SuggestSanitizedName(builtin_position_.per_vertex_var_id,
                                 "gl_Position");
+    ast::DecorationList decos;
+    if (!hlsl_style_pipeline_io_) {
+      // When doing HLSL-style pipeline IO, the decoration goes on the
+      // parameter, not the variable.
+      decos.push_back(
+          create<ast::BuiltinDecoration>(Source{}, ast::Builtin::kPosition));
+    }
     auto* var = MakeVariable(
         builtin_position_.per_vertex_var_id,
         enum_converter_.ToStorageClass(builtin_position_.storage_class),
         ConvertType(builtin_position_.position_member_type_id), false, nullptr,
-        ast::DecorationList{
-            create<ast::BuiltinDecoration>(Source{}, ast::Builtin::kPosition),
-        });
+        std::move(decos));
 
     builder_.AST().AddGlobalVariable(var);
   }
diff --git a/src/reader/spirv/parser_impl_module_var_test.cc b/src/reader/spirv/parser_impl_module_var_test.cc
index 729d3293f6..11f2328619 100644
--- a/src/reader/spirv/parser_impl_module_var_test.cc
+++ b/src/reader/spirv/parser_impl_module_var_test.cc
@@ -4841,6 +4841,68 @@ TEST_F(SpvModuleScopeVarParserTest,
   EXPECT_EQ(got, expected) << got;
 }
 
+TEST_F(SpvModuleScopeVarParserTest,
+       BuiltinPosition_BuiltIn_Position_MapsToVec4) {
+  // In Vulkan SPIR-V, Position is the first member of gl_PerVertex
+  const std::string assembly = PerVertexPreamble() + R"(
+  %main = OpFunction %void None %voidfn
+  %entry = OpLabel
+  OpReturn
+  OpFunctionEnd
+)";
+  auto p = parser(test::Assemble(assembly));
+
+  // TODO(crbug.com/tint/508): Remove this when everything is converted
+  // to HLSL style pipeline IO.
+  p->SetHLSLStylePipelineIO();
+
+  ASSERT_TRUE(p->Parse()) << p->error() << assembly;
+  EXPECT_TRUE(p->error().empty());
+
+  const auto got = p->program().to_str();
+  const std::string expected = R"(Module{
+  Struct main_out {
+    StructMember{[[ BuiltinDecoration{position}
+ ]] gl_Position: __vec_4__f32}
+  }
+  Variable{
+    gl_Position
+    private
+    undefined
+    __vec_4__f32
+  }
+  Function main_1 -> __void
+  ()
+  {
+    Return{}
+  }
+  Function main -> __type_name_main_out
+  StageDecoration{vertex}
+  ()
+  {
+    Call[not set]{
+      Identifier[not set]{main_1}
+      (
+      )
+    }
+    Return{
+      {
+        TypeConstructor[not set]{
+          __type_name_main_out
+          Identifier[not set]{gl_Position}
+        }
+      }
+    }
+  }
+}
+)";
+  EXPECT_EQ(got, expected) << got;
+}
+
+// TODO(dneto): pipeline IO: gl_Position, with initializer
+// TODO(dneto): pipeline IO: read PointSize
+// TODO(dneto): pipeline IO: write PointSize
+
 // TODO(dneto): pipeline IO: flatten structures, and distribute locations
 
 }  // namespace