He was thought to possibly be next heading to the UFC after his contract with Strikeforce expired. Instead, Hieron signed with Bellator to compete in their fourth season welterweight tournament. He was set to face Steve Carl in the tournament's first round. Carl was injured and replaced by Anthony Lapsley. Hieron defeated Lapsley via first-round technical submission due to a rear-naked choke. The stoppage was a controversial one, but Hieron advanced nonetheless.
He next faced Brent Weedman in the semifinals, but the victory was once again controversial as many fans scored the fight for Weedman. Regardless, Hieron moved on to the finals to face the undefeated judo practitioner Rick Hawn. He defeated Hawn via a close split decision. Hieron next faced Ben Askren for the Bellator welterweight title, losing via a hard-fought and bloody split decision.
Hieron next faced Romario da Silva in the LFC, winning via second round D'arce choke submission. Hieron then stepped in on short notice to return to the UFC and replace Josh Koscheck in a rematch against Jake Ellenberger. Ellenberger defeated Hieron via a close and relatively uneventful unanimous decision.
Hieron next signed to face dangerous striker and grappler Erick Silva in February 2013. Unfortunately Silva was injured and replaced on about a month's notice by Strikeforce veteran and UFC newcomer, the wrestler Tyron Woodley. Woodley knocked Hieron out very quickly and afterwards Hieron was again cut from the UFC.
About a year later Hieron retired from the sport of mixed martial arts in March 2014.
- Jonathan Goulet vs. Jay Hieron - The fight was Jonathan Goulet's UFC debut and it was widely considered to be one of the bloodiest fights in UFC/MMA history.
- Jay Hieron vs. Brad Blackburn - The fight was considered a mild upset for Brad Blackburn with the quick knockout victory.
- Jay Hieron vs. Delson Heleno - The fight was the finals of the IFL welterweight grand prix for the vacant IFL welterweight title.
- Joe Riggs vs. Jay Hieron - The fight was considered to be a number-one contender bout for the Strikeforce welterweight title, with the winner, Hieron, presumably set to face champion Nick Diaz.
- Jay Hieron vs. Anthony Lapsley - The fight was in the quarterfinals of the Bellator fourth season welterweight tournament and it was the Bellator debut of both men.
- Brent Weedman vs. Jay Hieron - The fight was in the semifinals of the Bellator fourth season welterweight tournament. It was a controversial decision victory for Jay Hieron.
- Jay Hieron vs. Rick Hawn - The fight was in the finals of the Bellator season four welterweight tournament. The fight was Rick Hawn's first loss.
- Ben Askren vs. Jay Hieron - The fight was for the Bellator welterweight title with Ben Askren defending.
- Jake Ellenberger vs. Jay Hieron 2 - Hieron was a replacement for an injured Josh Koscheck. It was his return to the Octagon after seven years away.