Icelandic behemoth Hafthor Bjornsson -- AKA "The Mountain" from "Game of Thrones" -- is calling it QUITS ... retiring from Strongman competitions after another dominant victory!
The 31-year-old legend made the stunning announcement Sunday after being crowned Iceland's Strongest Man for the 10th consecutive year!
"Great way to finish my strongman career," Bjornsson said ... "Now a different journey starts and I’m super excited."
Bjornsson definitely went out with a bang -- winning all 6 events at the Iceland competition ... a clean sweep.
But he's not completely shutting the door on a comeback -- saying he'll continue to train .