Junior dos Santos is trading the Octagon for a triangle ... 'cause we've learned the former UFC champ has inked a deal with Triad Combat -- and will fight early next year!
And, 37-year-old dos Santos' opponent?? Kubrat Pulev ... the 40-year-old boxer who's faced the likes of Anthony Joshua.
Pulev lost to the British star ... but he's got a 28-2 professional boxing record and just dominated MMA star Frank Mir in a Triad fight last month -- leaving him knocked out on his feet.
We're told the exact date isn't yet set in stone ... but the fight -- a mix of boxing/dirty boxing in a triangular-shaped ring -- will go down in late February.
Dos Santos was released by the UFC earlier this year after facing a murderers row of opponents ... including Francis Ngannou and Ciryl Gane.
Junior has always been known for his hands ... and clearly believes he ain't done.
JDS and Pulev aren't the only notable fighters on the card ... MMA legend 44-year-old Vitor Belfort, who recently beat Evander Holyfield, is fighting former light heavyweight boxing champ, 39-year-old Chad Dawson.
Like the Triad event in November, the teams are split up by craft ... Boxers vs. MMA fighters. Shannon Briggs will again act as captain for the pugilists ... while Rampage Jackson for the MMA team.