diff --git a/protoc-gen-openapiv2/internal/genopenapi/BUILD.bazel b/protoc-gen-openapiv2/internal/genopenapi/BUILD.bazel
index 6af63ffc8af..66ea1948725 100644
--- a/protoc-gen-openapiv2/internal/genopenapi/BUILD.bazel
+++ b/protoc-gen-openapiv2/internal/genopenapi/BUILD.bazel
@@ -49,6 +49,7 @@ go_test(
         "template_test.go",
         "types_test.go",
     ],
+    data = glob(["testdata/**"]),
     embed = [":genopenapi"],
     deps = [
         "//internal/descriptor",
diff --git a/protoc-gen-openapiv2/internal/genopenapi/generator.go b/protoc-gen-openapiv2/internal/genopenapi/generator.go
index 5087af77614..5ec31de63c7 100644
--- a/protoc-gen-openapiv2/internal/genopenapi/generator.go
+++ b/protoc-gen-openapiv2/internal/genopenapi/generator.go
@@ -150,20 +150,45 @@ func (po openapiPathsObject) MarshalYAML() (interface{}, error) {
 	pathObjectNode.Kind = yaml.MappingNode
 
 	for _, pathData := range po {
-		var pathNode, pathItemObjectNode yaml.Node
+		var pathNode yaml.Node
 
 		pathNode.SetString(pathData.Path)
-		b, err := yaml.Marshal(pathData.PathItemObject)
+		pathItemObjectNode, err := pathData.PathItemObject.toYAMLNode()
 		if err != nil {
 			return nil, err
 		}
-		pathItemObjectNode.SetString(string(b))
-		pathObjectNode.Content = append(pathObjectNode.Content, &pathNode, &pathItemObjectNode)
+		pathObjectNode.Content = append(pathObjectNode.Content, &pathNode, pathItemObjectNode)
 	}
 
 	return pathObjectNode, nil
 }
 
+// We can simplify this implementation once the go-yaml bug is resolved. See: https://github.com/go-yaml/yaml/issues/643.
+//
+//	func (pio *openapiPathItemObject) toYAMLNode() (*yaml.Node, error) {
+//		var node yaml.Node
+//		if err := node.Encode(pio); err != nil {
+//			return nil, err
+//		}
+//		return &node, nil
+//	}
+func (pio *openapiPathItemObject) toYAMLNode() (*yaml.Node, error) {
+	var doc yaml.Node
+	var buf bytes.Buffer
+	ec := yaml.NewEncoder(&buf)
+	ec.SetIndent(2)
+	if err := ec.Encode(pio); err != nil {
+		return nil, err
+	}
+	if err := yaml.Unmarshal(buf.Bytes(), &doc); err != nil {
+		return nil, err
+	}
+	if len(doc.Content) == 0 {
+		return nil, errors.New("unexpected number of yaml nodes")
+	}
+	return doc.Content[0], nil
+}
+
 func (so openapiInfoObject) MarshalJSON() ([]byte, error) {
 	type alias openapiInfoObject
 	return extensionMarshalJSON(alias(so), so.extensions)
diff --git a/protoc-gen-openapiv2/internal/genopenapi/generator_test.go b/protoc-gen-openapiv2/internal/genopenapi/generator_test.go
index 59194b3f38f..559259ca2ea 100644
--- a/protoc-gen-openapiv2/internal/genopenapi/generator_test.go
+++ b/protoc-gen-openapiv2/internal/genopenapi/generator_test.go
@@ -1,11 +1,13 @@
 package genopenapi_test
 
 import (
+	"os"
 	"reflect"
 	"sort"
 	"strings"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/internal/descriptor"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2/internal/genopenapi"
 	"gopkg.in/yaml.v3"
@@ -120,6 +122,54 @@ func TestGenerateExtension(t *testing.T) {
 	}
 }
 
+func TestGenerateYAML(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name           string
+		inputProtoText string
+		wantYAML       string
+	}{
+		{
+			// It tests https://github.com/grpc-ecosystem/grpc-gateway/issues/3557.
+			name:           "path item object",
+			inputProtoText: "testdata/generator/path_item_object.prototext",
+			wantYAML:       "testdata/generator/path_item_object.swagger.yaml",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			b, err := os.ReadFile(tt.inputProtoText)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var req pluginpb.CodeGeneratorRequest
+			if err := prototext.Unmarshal(b, &req); err != nil {
+				t.Fatal(err)
+			}
+
+			resp := requireGenerate(t, &req, genopenapi.FormatYAML, false, true)
+			if len(resp) != 1 {
+				t.Fatalf("invalid count, expected: 1, actual: %d", len(resp))
+			}
+			got := resp[0].GetContent()
+
+			want, err := os.ReadFile(tt.wantYAML)
+			if err != nil {
+				t.Fatal(err)
+			}
+			diff := cmp.Diff(string(want), got)
+			if diff != "" {
+				t.Fatalf("content not match\n%s", diff)
+			}
+		})
+	}
+}
+
 func requireGenerate(
 	tb testing.TB,
 	req *pluginpb.CodeGeneratorRequest,
diff --git a/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.prototext b/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.prototext
new file mode 100644
index 00000000000..63a641be2b9
--- /dev/null
+++ b/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.prototext
@@ -0,0 +1,32 @@
+file_to_generate:  "your/service/v1/your_service.proto"
+proto_file:  {
+ name:  "your/service/v1/your_service.proto"
+ package:  "your.service.v1"
+ message_type:  {
+  name:  "StringMessage"
+  field:  {
+   name:  "value"
+   number:  1
+   label:  LABEL_OPTIONAL
+   type:  TYPE_STRING
+   json_name:  "value"
+  }
+ }
+ service:  {
+  name:  "YourService"
+  method:  {
+   name:  "Echo"
+   input_type:  ".your.service.v1.StringMessage"
+   output_type:  ".your.service.v1.StringMessage"
+   options:  {
+    [google.api.http]:  {
+     post:  "/api/echo"
+    }
+   }
+  }
+ }
+ options:  {
+  go_package:  "github.com/yourorg/yourprotos/gen/go/your/service/v1"
+ }
+ syntax:  "proto3"
+}
diff --git a/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.swagger.yaml b/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.swagger.yaml
new file mode 100644
index 00000000000..217ea3b248e
--- /dev/null
+++ b/protoc-gen-openapiv2/internal/genopenapi/testdata/generator/path_item_object.swagger.yaml
@@ -0,0 +1,32 @@
+swagger: "2.0"
+info:
+  title: your/service/v1/your_service.proto
+  version: version not set
+tags:
+  - name: YourService
+consumes:
+  - application/json
+produces:
+  - application/json
+paths:
+  /api/echo:
+    post:
+      operationId: YourService_Echo
+      responses:
+        "200":
+          description: A successful response.
+          schema:
+            $ref: '#/definitions/StringMessage'
+      parameters:
+        - name: value
+          in: query
+          required: false
+          type: string
+      tags:
+        - YourService
+definitions:
+  StringMessage:
+    type: object
+    properties:
+      value:
+        type: string