Bonnar was highly regarded by fight fans, with notable bouts against the likes Rashad Evans, Anderson Silva and Jon Jones.
However, it was his stint on the original series of the reality show The Ultimate Fighter that helped Bonnar to cement his place in MMA history.
His fight with fellow contestant Forrest Griffin in the televised final was widely credited as helping to save a financially struggling UFC, with the back-and-forth slugfest drawing a legion of new fans as it aired live, propelling the sport into the living rooms of a generation of new fans.
While he eventually lost the bout - regarded by many as one of the greatest mixed martial arts fights of all time - both contestants received a lucrative contract with the company.
UFC president Dana White was one of the first to share his condolences.
"Stephan Bonnar was one of the most important fighters to ever compete in the Octagon," stated White in a release announcing Bonnar's death.
"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 faced a series of personal challenges in recent years, most recently the destruction of his Las Vegas home in April due to fire. Bonnar's wife, Andrea Brown, posted on Facebook that the couple "lost everything" in the huge blaze.
He briefly came out of retirement to fight fellow former UFC star Tito Ortiz at Bellator 131 in 2014, before re-retiring and finding success on the independent professional wrestling scene.
