William John Hahn (November 5, 1841 – September 23, 1902) was an American lawyer and politician from Minnesota.
In 1866 Hahn moved to Philadelphia and read law in the office of Judge Morris and at Pennsylvania College.
In March 1881 he was appointed by Governor John S. Pillsbury as Minnesota Attorney General.
Hahn was married on September 16, 1868, to Emily Loretta Martin of Mifflin County, Pennsylvania.
Hang subsequently married Matilda Richardson of Minneapolis, who survived him along with three daughters.