Admiral Sir William Cornwallis, GCB (February 10, 1744 – July 5, 1819) is a Royal Navy officer.

He was the brother of Charles Cornwallis, the 1st Marquess Cornwallis, British commander at the siege of Yorktown. Cornwallis took part in a number of decisive battles including the Siege of Louisbourg in 1758 and the Battle of the Saintes but is best known as a friend of Lord Nelson and as the commander-in-chief of the Channel Fleet during the Napoleonic Wars. He is depicted in the Horatio Hornblower novel, Hornblower and the Hotspur.