James Thomas Hieronymus (born July 29, 1976) is an American professional mixed martial arts (MMA) fighter. He was the last welterweight champion of the International Fight League. He is currently under contract to Bellator Fighting Championships.

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 Xtreme Couture. 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, New Jersey.

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

With the collapse of Affliction in July 2009, Hieron moved on to Strikeforce.

Hieron's first fight with his latest promotion was set to be against fellow UFC veteran Nick Diaz at Strikeforce: Carano vs. Cyborg for the inaugural Strikeforce welterweight title. However, Diaz missed a pre-fight drug test mandated by the California State Athletic Commission and was denied a license to compete. Jesse Taylor was named to replace Diaz and the fight was changed to a non-title bout. Hieron won by unanimous decision. He had his heart set on a title shot against Nick Diaz, but has stated that he would be comfortable fighting any top level opponent, as long as the fight, and name is helping him move forward in his career.