diff --git a/include/vkd3d_shader.h b/include/vkd3d_shader.h
index 4acb6224..d4756810 100644
--- a/include/vkd3d_shader.h
+++ b/include/vkd3d_shader.h
@@ -553,6 +553,30 @@ enum vkd3d_shader_parameter_name
      * \since 1.13
      */
     VKD3D_SHADER_PARAMETER_NAME_ALPHA_TEST_REF,
+    /**
+     * Whether to use flat interpolation for fragment shader colour inputs.
+     * If the value is nonzero, inputs whose semantic usage is COLOR will use
+     * flat interpolation instead of linear.
+     * This parameter is ignored if the shader model is 4 or greater, since only
+     * shader model 3 and below do not specify the interpolation mode in the
+     * shader bytecode.
+     *
+     * This parameter can be used to implement fixed function shade mode, as
+     * present in Direct3D versions up to 9, if the target environment does not
+     * support shade mode as part of its own fixed-function API (as Vulkan and
+     * core OpenGL).
+     *
+     * The data type for this parameter must be
+     * VKD3D_SHADER_PARAMETER_DATA_TYPE_UINT32.
+     *
+     * The default value is zero, i.e. use linear interpolation.
+     *
+     * Only VKD3D_SHADER_PARAMETER_TYPE_IMMEDIATE_CONSTANT is supported in this
+     * version of vkd3d-shader.
+     *
+     * \since 1.13
+     */
+    VKD3D_SHADER_PARAMETER_NAME_FLAT_INTERPOLATION,
 
     VKD3D_FORCE_32_BIT_ENUM(VKD3D_SHADER_PARAMETER_NAME),
 };
diff --git a/libs/vkd3d-shader/ir.c b/libs/vkd3d-shader/ir.c
index 6c98a32e..e0ac6322 100644
--- a/libs/vkd3d-shader/ir.c
+++ b/libs/vkd3d-shader/ir.c
@@ -1723,7 +1723,33 @@ static void shader_instruction_normalise_io_params(struct vkd3d_shader_instructi
     }
 }
 
-static enum vkd3d_result vsir_program_normalise_io_registers(struct vsir_program *program)
+static bool use_flat_interpolation(const struct vsir_program *program,
+        struct vkd3d_shader_message_context *message_context)
+{
+    static const struct vkd3d_shader_location no_loc;
+    const struct vkd3d_shader_parameter1 *parameter;
+
+    if (!(parameter = vsir_program_get_parameter(program, VKD3D_SHADER_PARAMETER_NAME_FLAT_INTERPOLATION)))
+        return false;
+
+    if (parameter->type != VKD3D_SHADER_PARAMETER_TYPE_IMMEDIATE_CONSTANT)
+    {
+        vkd3d_shader_error(message_context, &no_loc, VKD3D_SHADER_ERROR_VSIR_NOT_IMPLEMENTED,
+                "Unsupported flat interpolation parameter type %#x.\n", parameter->type);
+        return false;
+    }
+    if (parameter->data_type != VKD3D_SHADER_PARAMETER_DATA_TYPE_UINT32)
+    {
+        vkd3d_shader_error(message_context, &no_loc, VKD3D_SHADER_ERROR_VSIR_INVALID_DATA_TYPE,
+                "Invalid flat interpolation parameter data type %#x.\n", parameter->data_type);
+        return false;
+    }
+
+    return parameter->u.immediate_constant.u.u32;
+}
+
+static enum vkd3d_result vsir_program_normalise_io_registers(struct vsir_program *program,
+        struct vkd3d_shader_message_context *message_context)
 {
     struct io_normaliser normaliser = {program->instructions};
     struct vkd3d_shader_instruction *ins;
@@ -1786,6 +1812,18 @@ static enum vkd3d_result vsir_program_normalise_io_registers(struct vsir_program
         return VKD3D_ERROR_OUT_OF_MEMORY;
     }
 
+    if (program->shader_version.type == VKD3D_SHADER_TYPE_PIXEL
+            && program->shader_version.major < 4 && use_flat_interpolation(program, message_context))
+    {
+        for (i = 0; i < program->input_signature.element_count; ++i)
+        {
+            struct signature_element *element = &program->input_signature.elements[i];
+
+            if (!ascii_strcasecmp(element->semantic_name, "COLOR"))
+                element->interpolation_mode = VKD3DSIM_CONSTANT;
+        }
+    }
+
     normaliser.phase = VKD3DSIH_INVALID;
     for (i = 0; i < normaliser.instructions.count; ++i)
         shader_instruction_normalise_io_params(&normaliser.instructions.elements[i], &normaliser);
@@ -6591,7 +6629,7 @@ enum vkd3d_result vsir_program_normalise(struct vsir_program *program, uint64_t
                 return result;
         }
 
-        if ((result = vsir_program_normalise_io_registers(program)) < 0)
+        if ((result = vsir_program_normalise_io_registers(program, message_context)) < 0)
             return result;
 
         if ((result = instruction_array_normalise_flat_constants(program)) < 0)