William Sebastian Cohen (born 28 August 1940) is an author and American politician from the U.S. state of Maine. A Republican, Cohen served as Secretary of Defense (1997–2001) under Democratic President Bill Clinton.

Cohen was born in Bangor, Maine. His mother, Clara, was of Protestant Irish ancestry, while his father, Reuben Cohen, was a Russian Jewish immigrant; the two owned the Bangor Rye Bread Co. Cohen attended Hebrew school as a child.

After graduating from Bangor High School in 1958, Cohen attended Bowdoin College, graduating cum laude with a Bachelor of Arts degree in Latin in 1962.