William Nicholas Leonard (January 12, 1916 – August 21, 2005) was an American aviator, a flying ace of World War II, and a rear admiral in the United States Navy.

"In the words of Barrett Tillman, World War II aviation historian, [Leonard] is a 'national treasure' because he is so generous with his time and knowledge, records, and photos.

"[1] After the war, he became a test pilot and commander, including of carrier groups.

Two of his brothers also became high-ranking officers: Army Major General Charles F. Leonard, Jr. and Army Air Forces Lieutenant Colonel John Wallis Leonard, who was killed in action in World War II.

William Leonard, his father and two brothers are all interred in Arlington National Cemetery.