diff --git a/apispec/ext/marshmallow/swagger.py b/apispec/ext/marshmallow/swagger.py
index ea977448..3f676a1e 100644
--- a/apispec/ext/marshmallow/swagger.py
+++ b/apispec/ext/marshmallow/swagger.py
@@ -241,6 +241,9 @@ def field2property(field, spec=None, use_refs=True, dump=True, name=None):
     if field.dump_only:
         ret['readOnly'] = True
 
+    if field.allow_none:
+        ret['x-nullable'] = True
+
     ret.update(field2range(field))
     ret.update(field2length(field))
 
diff --git a/tests/test_swagger.py b/tests/test_swagger.py
index d31ad4d3..b460b653 100644
--- a/tests/test_swagger.py
+++ b/tests/test_swagger.py
@@ -128,6 +128,10 @@ def test_field_with_additional_metadata(self):
         assert res['maxLength'] == 100
         assert res['minLength'] == 6
 
+    def test_field_with_allow_none(self):
+        field = fields.Str(allow_none=True)
+        res = swagger.field2property(field)
+        assert res['x-nullable'] is True
 
 class TestMarshmallowSchemaToModelDefinition: