The UFC broke the news Saturday, writing ... "The UFC family is saddened by the tragic passing of UFC Hall of Famer Stephan Bonnar Thursday from presumed heart complications while at work." Per reports, he apparently died a couple days ago ... an official cause of death is pending.

UFC goes on to mention SB's storied career, adding ... "Stephan Bonnar was one of the most important fighters to ever compete in the Octagon," said UFC President Dana White. "His fight with Forrest Griffin changed the sport forever, and he will never be forgotten. The fans loved him, related to him and he always gave them his best. He will be missed."

Bonnar was an OG in UFC ... competing in the very first season of 'The Ultimate Fighter' back in 2005 ... an appearance which saw him go all the way to the finale. Although he lost his Light Heavyweight fight against Forrest Griffin ... his bout ended up earning him a contract with UFC, where he went on to compete at a high level for several years.

He fought Griffin again in a rematch, and went on to fight some big names in the sport thereafter ... including Jon Jones, Mark Coleman, Krzysztof Soszynski, Igor Pokrajac, Kyle Kingsbury, Anderson Silva and others. His record with UFC was 15-9-0 ... and in 2012, he retired. The following year, Dana White announced Bonnar would be inducted into the HOF.

In addition to MMA fighting, Bonnar also wrestled professionally ... including for leagues like Impact Wrestling. He also fought with Bellator MMA post-UFC.

Bonnar's legacy will be that he was one of the catalysts to help UFC grow into what it is today ... and he goes down as one of the sport's greatest known fighters to date. He's survived by his wife, Andrea, and their son Griffin. He was 45.