diff --git a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/StructureGenerator.java b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/StructureGenerator.java
index 55d4daf67..71e1edbd7 100644
--- a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/StructureGenerator.java
+++ b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/StructureGenerator.java
@@ -207,6 +207,14 @@ private void writeProperties() {
             if (member.hasTrait(DefaultTrait.class)) {
 
                 defaultValue = getDefaultValue(writer, member);
+                // A `@default: null` member is nullable and resolves to None, so
+                // it needs `| None` like a member with no default. Documents are
+                // excluded: a null document default is a non-None Document(None),
+                // built via default_factory below.
+                if (!target.isDocumentShape()
+                        && member.expectTrait(DefaultTrait.class).toNode().isNullNode()) {
+                    writer.putContext("nullable", true);
+                }
                 if (target.isDocumentShape() || defaultValue.startsWith("list[") || defaultValue.startsWith("dict[")) {
                     writer.addStdlibImport("dataclasses", "field");
                     defaultKey = "default_factory";