William Cowper Alexander (May 20, 1806 – August 23, 1874) was an American lawyer, politician, and insurance executive.
His elder brother James Waddel Alexander (1804–1859) would also become a Presbyterian theologian and minister.
Alexander's father was president of Hampden-Sydney College in Virginia before being called to serve as minister of the Third Presbyterian Church in Philadelphia in 1807.
In 1851 he succeeded Charles Smith Olden as a member of the New Jersey Senate from Mercer County, serving as president of that body for four terms.
His brother, Presbyterian minister James Waddel Alexander, had become the patron of Equitable founder Henry Baldwin Hyde, and many of the company's original directors were members of Alexander's Fifth Avenue Presbyterian Church.