William Bainbridge Castle (November 30, 1814 – February 28, 1872) was an American politician of the Whig Party who served as the 11th and final mayor of Ohio City from 1853 to 1854 and the 14th mayor of Cleveland, Ohio from 1855 to 1856. He moved back to Ontario and then to Ohio City in 1839. He entered politics and quickly rose to prominence as a member of the Ohio City Common Council. In 1853, he was elected the city's mayor and assisted in authoring the 1854 agreement to merge Cleveland and Ohio City. In 1855, he became mayor of Cleveland.