diff --git a/internal/apijson/decoder.go b/internal/apijson/decoder.go
index e1b21b7..68b7ed6 100644
--- a/internal/apijson/decoder.go
+++ b/internal/apijson/decoder.go
@@ -143,7 +143,9 @@ func (d *decoderBuilder) newTypeDecoder(t reflect.Type) decoderFunc {
 		return unmarshalerDecoder
 	}
 	if !d.root && reflect.PointerTo(t).Implements(reflect.TypeOf((*json.Unmarshaler)(nil)).Elem()) {
-		return indirectUnmarshalerDecoder
+		if _, ok := unionVariants[t]; !ok {
+			return indirectUnmarshalerDecoder
+		}
 	}
 	d.root = false
 
diff --git a/internal/apijson/json_test.go b/internal/apijson/json_test.go
index 72bc4c2..85cd2b5 100644
--- a/internal/apijson/json_test.go
+++ b/internal/apijson/json_test.go
@@ -261,6 +261,59 @@ func init() {
 	)
 }
 
+type MarshallingUnionStruct struct {
+	Union MarshallingUnion
+}
+
+func (r *MarshallingUnionStruct) UnmarshalJSON(data []byte) (err error) {
+	*r = MarshallingUnionStruct{}
+	err = UnmarshalRoot(data, &r.Union)
+	return
+}
+
+func (r MarshallingUnionStruct) MarshalJSON() (data []byte, err error) {
+	return MarshalRoot(r.Union)
+}
+
+type MarshallingUnion interface {
+	marshallingUnion()
+}
+
+type MarshallingUnionA struct {
+	Boo string `json:"boo"`
+}
+
+func (MarshallingUnionA) marshallingUnion() {}
+
+func (r *MarshallingUnionA) UnmarshalJSON(data []byte) (err error) {
+	return UnmarshalRoot(data, r)
+}
+
+type MarshallingUnionB struct {
+	Foo string `json:"foo"`
+}
+
+func (MarshallingUnionB) marshallingUnion() {}
+
+func (r *MarshallingUnionB) UnmarshalJSON(data []byte) (err error) {
+	return UnmarshalRoot(data, r)
+}
+
+func init() {
+	RegisterUnion(
+		reflect.TypeOf((*MarshallingUnion)(nil)).Elem(),
+		"",
+		UnionVariant{
+			TypeFilter: gjson.JSON,
+			Type:       reflect.TypeOf(MarshallingUnionA{}),
+		},
+		UnionVariant{
+			TypeFilter: gjson.JSON,
+			Type:       reflect.TypeOf(MarshallingUnionB{}),
+		},
+	)
+}
+
 var tests = map[string]struct {
 	buf string
 	val interface{}
@@ -489,6 +542,15 @@ var tests = map[string]struct {
 		ComplexUnionStruct{Union: ComplexUnionTypeB{Baz: 12, Type: TypeB("b")}},
 	},
 
+	"marshalling_union_a": {
+		`{"boo":"hello"}`,
+		MarshallingUnionStruct{Union: MarshallingUnionA{Boo: "hello"}},
+	},
+	"marshalling_union_b": {
+		`{"foo":"hi"}`,
+		MarshallingUnionStruct{Union: MarshallingUnionB{Foo: "hi"}},
+	},
+
 	"unmarshal": {
 		`{"foo":"hello"}`,
 		&UnmarshalStruct{Foo: "hello", prop: true},
diff --git a/internal/apijson/registry.go b/internal/apijson/registry.go
index fcc518b..2ea00ae 100644
--- a/internal/apijson/registry.go
+++ b/internal/apijson/registry.go
@@ -13,6 +13,7 @@ type UnionVariant struct {
 }
 
 var unionRegistry = map[reflect.Type]unionEntry{}
+var unionVariants = map[reflect.Type]interface{}{}
 
 type unionEntry struct {
 	discriminatorKey string
@@ -24,4 +25,7 @@ func RegisterUnion(typ reflect.Type, discriminator string, variants ...UnionVari
 		discriminatorKey: discriminator,
 		variants:         variants,
 	}
+	for _, variant := range variants {
+		unionVariants[variant.Type] = typ
+	}
 }