Brock Lesnar failed a drug test on the day of his fight at UFC 200 against Mark Hunt, his second positive test in as many weeks.
The WWE wrestler, a graduate of the University of Minnesota, gave an in-competition sample on July 9 that fell foul of anti-doping rules, the UFC announced on Tuesday.
The USADA, which administers the UFC's anti-doping policy, informed the organization and will rule on an "appropriate adjudication" on Lesnar's case.
Lesnar failed an out-of-competition drug test that was taken on June 28.
"It is important to note that, under the UFC Anti-Doping Policy, there is a full fair legal review process that is afforded to all athletes before any sanctions are imposed," a UFC statement said.
"The Nevada State Athletic Commission also retains jurisdiction over this matter as the sample collection was performed at UFC 200 in Las Vegas."
Lesnar was the showpiece attraction at UFC 200, beating Mark Hunt via unanimous decision four years after making his last appearance in the Octagon.
He is due to perform at WWE Summerslam next month where he will take on superstar Randy Orton.