Fraser is the most decorated CrossFitter who's ever lived ... he won the CrossFit Games the last FIVE years -- and in most of the events, he absolutely dusted the competition.

But, the 31-year-old said in his goodbye to the sport Tuesday that he's now ready to move on to "the next chapter" of his life.

"From now on, decisions I make won't be solely based off how they affect my performance, but how they affect my family, friends, health and happiness," Fraser said.

"I look forward to remaining a part of the Crossfit community, I just won't be doing it from the competition floor any longer."

Fraser -- who grew up a weight lifter with goals of winning Olympic gold -- jumped into the Crossfit world in 2012 and eventually overtook Rich Froning as the sport's GOAT.

But Tuesday, Fraser said that journey took a massive toll on him ... explaining he was so obsessed with dominating the sport, it sucked most of the joy out of his life.

"I’ve passed up vacations, bachelor parties, and more dates with [my wife] Sammy than I can count," Fraser said, "all so I wouldn’t miss a single training session or a full night of sleep."

Fraser says he's not sure where his future lies from here ... but he did say he'll likely begin revealing some of his training techniques to help others grow in the sport.

"Thank you for allowing me this opportunity," Fraser said, "and a special thanks to everyone who’s helped me as a sponsor, training partner, coach, or friend."