Donshay White, fighting his first bout since November 2015, was up against Ricky Muse during Hardrock MMA 90 in Louisville, Kentucky.
The fight was stopped after 2:22 in the second round after White failed to protect himself against Muse.
But at the time the fighter was responsive to referee Gary Copeland.
Following the bout, it is reported White looked exhausted and needed medical attention, according to BluegrassMMA.
It is understood he was able to make it backstage but once there he collapsed and became unresponsive.
He received CPR and was transported to a nearby hospital but later died.
The cause of death is not yet known.
Video shows his previous fight during Hardrock MMA 76 in November 2015 against John Johnson.
He won that fight after knocking out his opponent.
His death follows that of Tim Hague, the UFC fighter-turned-boxer who died following a knockout loss to Adam Braidwood last month.