diff --git a/handwritten/spanner/src/partial-result-stream.ts b/handwritten/spanner/src/partial-result-stream.ts
index 18f492e577c..dde0c01de24 100644
--- a/handwritten/spanner/src/partial-result-stream.ts
+++ b/handwritten/spanner/src/partial-result-stream.ts
@@ -121,6 +121,22 @@ export interface Row extends Array<Field> {
   toJSON(options?: JSONOptions): Json;
 }
 
+/**
+ * Row implementation extending Array to provide a shared, non-enumerable
+ * toJSON method without per-row closures or Object.setPrototypeOf overhead.
+ */
+class RowImpl extends Array<Field> implements Row {
+  toJSON(options?: JSONOptions): Json {
+    return codec.convertFieldsToJson(this, options);
+  }
+}
+Object.defineProperty(RowImpl.prototype, 'constructor', {
+  value: Array,
+  writable: true,
+  configurable: true,
+  enumerable: false,
+});
+
 /**
  * @callback PartialResultStream~rowCallback
  * @param {Row|object} row The row data.
@@ -458,7 +474,7 @@ export class PartialResultStream extends Transform implements ResultEvents {
    */
   private _createRow(values: Value[]): Row {
     const len = values.length;
-    const fields = new Array(len);
+    const fields = new RowImpl(len);
     const decoders = this._decoders;
     const classFields = this._fields;
 
@@ -469,13 +485,7 @@ export class PartialResultStream extends Transform implements ResultEvents {
       };
     }
 
-    Object.defineProperty(fields, 'toJSON', {
-      value: (options?: JSONOptions): Json => {
-        return codec.convertFieldsToJson(fields, options);
-      },
-    });
-
-    return fields as Row;
+    return fields;
   }
   /**
    * Attempts to merge chunked values together.
diff --git a/handwritten/spanner/test/partial-result-stream.ts b/handwritten/spanner/test/partial-result-stream.ts
index 1aafbfca6a8..f1a0169e51b 100644
--- a/handwritten/spanner/test/partial-result-stream.ts
+++ b/handwritten/spanner/test/partial-result-stream.ts
@@ -200,6 +200,45 @@ describe('PartialResultStream', () => {
       stream.write(RESULT);
     });
 
+    it('should create rows with shared prototype and non-enumerable toJSON', done => {
+      const rows: prs.Row[] = [];
+      stream.on('error', done).on('data', row => {
+        rows.push(row);
+        if (rows.length === 2) {
+          try {
+            const [row1, row2] = rows;
+            assert.strictEqual(Array.isArray(row1), true);
+            assert.strictEqual(row1 instanceof Array, true);
+            assert.strictEqual(row1.constructor, Array);
+            assert.strictEqual(Array.isArray(row2), true);
+            assert.strictEqual(row2 instanceof Array, true);
+            assert.strictEqual(row2.constructor, Array);
+
+            // toJSON must be non-enumerable
+            assert.strictEqual(Object.keys(row1).includes('toJSON'), false);
+            assert.strictEqual(
+              Object.prototype.propertyIsEnumerable.call(row1, 'toJSON'),
+              false,
+            );
+
+            // toJSON must be shared on the prototype, not created as a per-row closure
+            assert.strictEqual(row1.toJSON, row2.toJSON);
+
+            // toJSON should correctly serialize the row
+            const json1 = row1.toJSON();
+            const expectedJson = codec.convertFieldsToJson(row1);
+            assert.deepStrictEqual(json1, expectedJson);
+            done();
+          } catch (error) {
+            done(error);
+          }
+        }
+      });
+
+      stream.write(RESULT);
+      stream.write({values: [convertToIValue(VALUE)]});
+    });
+
     it('should emit rows as JSON', done => {
       const jsonOptions = {};
       const stream = new PartialResultStream({json: true, jsonOptions});