John Sammon McKiernan (October 15, 1911 – March 9, 1997) was an American politician who served as the Lieutenant Governor of Rhode Island from 1947 to 1950 and 1951–1956 and briefly the 62nd Governor of Rhode Island (1950–1951).

As lieutenant governor and presiding officer of the state Senate in 1949, McKiernan cast the tie-breaking vote that resulted in the adoption of new rules effectively giving Democrats control of the Senate's legislative committees for the first time in more than a decade.

After Governor Pastore left office early to become a United States senator, following a special election to replace Senator J. Howard McGrath who was appointed United States Attorney General, McKiernan became governor and served for only 15 days from December 19, 1950, to January 2, 1951.

His was the shortest term as governor in the history of Rhode Island.

After winning re-election in 1952 and 1954, McKiernan resigned as lieutenant governor in 1956 to accept an appointment as an associate justice of the Rhode Island Superior Court.