James Hieronymous a.k.a Jay Hieron (born July 29, 1976 in Long Island, New York) is an American professional mixed martial arts (MMA) fighter. He was the last welterweight champion of the International Fight League. He's presently signed with Affliction. Hieron, a state wrestling champion at Freeport (N.Y.) High School and national Junior College Champion at Nassau (N.Y.) Community College, where he earned an Associates Degree, Hieron later attended Hofstra University. Before his senior year at Hofstra University, he tested positive for marijuana, and was unable to wrestle his senior year.

After being kicked off the wrestling team at Hofstra University, he began dealing drugs before being charged with a felony. He turned to boxing to let his aggressions out on the bags. Finding this to be a strong stress-reliever, and with the encouragement of some friends who trained in MMA, Hieron combined his wrestling skills and burgeoning boxing prowess into a professional MMA career.

Hieron married, moved to Las Vegas, where he has lived since 2004, and now trains full-time as a member of the Anacondas. In March 2006, he won the Lockdown in Paradise in Lahaina, Hawaii, and in June 2006 won his IFL debut at the IFL Team Championships in Atlantic City, N.J.

Following the demise of the International Fight League due to financial concerns, Hieron signed with Affliction in August 2008.