Coleman, 55, described the experience from last Tuesday night on Instagram alongside a picture of himself in a hospital bed, writing, "Chest locked up, my arms went numb."
He later took himself to the hospital, where doctors discovered significant issues.
"There was complete blockage of a major artery. If I don't go, I'm probably not here today," Coleman wrote.
He said he will have a procedure Monday to put a stent in his artery.
Coleman was the first heavyweight champion in UFC history, defeating Dan Severn at UFC 12 in 1997. He was inducted into the UFC Hall of Fame in 2008 but continued competing until 2010, when he battled Randy Couture at UFC 109.
He also spent much of his career competing for Pride FC while producing a 16-10 career record in MMA.