William Walter "Bill" Mercer (born 1964) is an American attorney and politician serving as a member of the Montana House of Representatives from the 46th district. He previously served as the United States Attorney for the District of Montana, as well as principal associate deputy attorney general for the United States Department of Justice. Mercer was nominated by President George W. Bush as Associate Attorney General and served in the position in an acting capacity, but resigned before his confirmation hearing could take place.