"Our beloved flyweight fighter, Allan Nascimento, was found unresponsive after suffering an apparent heart attack in his sleep," the UFC said on social media.
"Despite the efforts of the responding medical team, he was pronounced deceased at the scene."
Nascimento had been with the UFC since 2021 and his last fight was a split-decision loss to American Mitch Raposo on June 20.
He held an MMA record of 22 wins and seven losses.