William Kirk Kaynor (November 29, 1884 – December 20, 1929) was a United States Representative from Massachusetts.

Born in Sanborn, Iowa, Kaynor attended the common schools of Spencer and Clear Lake.

He moved to Springfield, Massachusetts and engaged in the real estate and insurance business.

During the First World War, he attended the officers' training school at Camp Lee, Virginia, from July to November 1918.

He was elected as a Republican to the Seventy-first Congress and served from March 4, 1929, until his death in a United States Army Air Corps airplane accident at Bolling Field in Washington, D.C., on December 20, 1929.