diff --git a/src/reader/spirv/function.cc b/src/reader/spirv/function.cc
index a0de6f829a..3a6707ff1d 100644
--- a/src/reader/spirv/function.cc
+++ b/src/reader/spirv/function.cc
@@ -1041,7 +1041,7 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
         // 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);
+        store_type = param_type = ty_.Vector(ty_.F32(), 4);
         out_decos.emplace_back(
             create<ast::BuiltinDecoration>(source, ast::Builtin::kPosition));
       } else {
diff --git a/src/reader/spirv/parser_impl.cc b/src/reader/spirv/parser_impl.cc
index 04e2e5d363..95e50ad5b5 100644
--- a/src/reader/spirv/parser_impl.cc
+++ b/src/reader/spirv/parser_impl.cc
@@ -1250,6 +1250,8 @@ bool ParserImpl::EmitModuleScopeVariables() {
          (spirv_storage_class == SpvStorageClassOutput))) {
       // Skip emitting gl_PerVertex.
       builtin_position_.per_vertex_var_id = var.result_id();
+      builtin_position_.per_vertex_var_init_id =
+          var.NumInOperands() > 1 ? var.GetSingleWordInOperand(1) : 0u;
       continue;
     }
     switch (enum_converter_.ToStorageClass(spirv_storage_class)) {
@@ -1322,13 +1324,33 @@ bool ParserImpl::EmitModuleScopeVariables() {
       decos.push_back(
           create<ast::BuiltinDecoration>(Source{}, ast::Builtin::kPosition));
     }
-    auto* var = MakeVariable(
+    ast::Expression* ast_constructor = nullptr;
+    if (builtin_position_.per_vertex_var_init_id) {
+      // The initializer is complex.
+      const auto* init =
+          def_use_mgr_->GetDef(builtin_position_.per_vertex_var_init_id);
+      switch (init->opcode()) {
+        case SpvOpConstantComposite:
+        case SpvOpSpecConstantComposite:
+          ast_constructor = MakeConstantExpression(
+                                init->GetSingleWordInOperand(
+                                    builtin_position_.position_member_index))
+                                .expr;
+          break;
+        default:
+          return Fail() << "gl_PerVertex initializer too complex. only "
+                           "OpCompositeConstruct and OpSpecConstantComposite "
+                           "are supported: "
+                        << init->PrettyPrint();
+      }
+    }
+    auto* ast_var = MakeVariable(
         builtin_position_.per_vertex_var_id,
         enum_converter_.ToStorageClass(builtin_position_.storage_class),
-        ConvertType(builtin_position_.position_member_type_id), false, nullptr,
-        std::move(decos));
+        ConvertType(builtin_position_.position_member_type_id), false,
+        ast_constructor, std::move(decos));
 
-    builder_.AST().AddGlobalVariable(var);
+    builder_.AST().AddGlobalVariable(ast_var);
   }
   return success_;
 }
diff --git a/src/reader/spirv/parser_impl.h b/src/reader/spirv/parser_impl.h
index 062cb58c87..d303bc026c 100644
--- a/src/reader/spirv/parser_impl.h
+++ b/src/reader/spirv/parser_impl.h
@@ -478,6 +478,8 @@ class ParserImpl : Reader {
     /// The ID of the gl_PerVertex variable, if it was declared.
     /// We'll use this for the gl_Position variable instead.
     uint32_t per_vertex_var_id = 0;
+    /// The ID of the initializer to gl_PerVertex, if any.
+    uint32_t per_vertex_var_init_id = 0;
   };
   /// @returns info about the gl_Position builtin variable.
   const BuiltInPositionInfo& GetBuiltInPositionInfo() {
diff --git a/src/reader/spirv/parser_impl_module_var_test.cc b/src/reader/spirv/parser_impl_module_var_test.cc
index 11f2328619..e3ff63ce64 100644
--- a/src/reader/spirv/parser_impl_module_var_test.cc
+++ b/src/reader/spirv/parser_impl_module_var_test.cc
@@ -4899,7 +4899,106 @@ TEST_F(SpvModuleScopeVarParserTest,
   EXPECT_EQ(got, expected) << got;
 }
 
-// TODO(dneto): pipeline IO: gl_Position, with initializer
+TEST_F(SpvModuleScopeVarParserTest,
+       BuiltinPosition_BuiltIn_Position_Initializer) {
+  const std::string assembly = R"(
+    OpCapability Shader
+    OpMemoryModel Logical Simple
+    OpEntryPoint Vertex %main "main" %1
+
+    OpMemberDecorate %10 0 BuiltIn Position
+    OpMemberDecorate %10 1 BuiltIn PointSize
+    OpMemberDecorate %10 2 BuiltIn ClipDistance
+    OpMemberDecorate %10 3 BuiltIn CullDistance
+    %void = OpTypeVoid
+    %voidfn = OpTypeFunction %void
+    %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+    %uint = OpTypeInt 32 0
+    %uint_0 = OpConstant %uint 0
+    %uint_1 = OpConstant %uint 1
+    %arr = OpTypeArray %float %uint_1
+    %10 = OpTypeStruct %v4float %float %arr %arr
+    %11 = OpTypePointer Output %10
+
+    %float_1 = OpConstant %float 1
+    %float_2 = OpConstant %float 2
+    %float_3 = OpConstant %float 3
+    %float_4 = OpConstant %float 4
+    %float_5 = OpConstant %float 5
+    %float_6 = OpConstant %float 6
+    %float_7 = OpConstant %float 7
+
+    %init_pos = OpConstantComposite %v4float %float_1 %float_2 %float_3 %float_4
+    %init_clip = OpConstantComposite %arr %float_6
+    %init_cull = OpConstantComposite %arr %float_7
+    %init_per_vertex = OpConstantComposite %10 %init_pos %float_5 %init_clip %init_cull
+
+    %1 = OpVariable %11 Output %init_per_vertex
+
+    %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
+    {
+      TypeConstructor[not set]{
+        __vec_4__f32
+        ScalarConstructor[not set]{1.000000}
+        ScalarConstructor[not set]{2.000000}
+        ScalarConstructor[not set]{3.000000}
+        ScalarConstructor[not set]{4.000000}
+      }
+    }
+  }
+  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: read PointSize
 // TODO(dneto): pipeline IO: write PointSize