Daniel Cormier added the UFC heavyweight championship to his light heavyweight title after he stopped Stipe Miocic in dramatic fashion during the first round of their UFC 226 superfight.
Cormier (21-1-1NC) flattened the UFC's long-reigning heavyweight champ with a right uppercut out of a clinch.
He finished Miocic (18-3) on the ground with 27 seconds left in the round by landing several shots before the referee stepped in.
Cormier became the second fighter in UFC history to hold two championship belts simultaneously.
Conor McGregor was the UFC's featherweight champ in 2016 when he took the lightweight title from Eddie Alvarez.
"I was a heavyweight for a long time, and I left the division," Cormier said. "I never knew what I could become, but tonight I got the answer. I'm a two-division champion, baby!"
The 39-year-old Cormier has never lost to anyone except Jon Jones, and his five-inch height disadvantage against Miocic was no problem; he actually weighed a few pounds more than his opponent at weigh-ins on Friday.
Cormier's move up to challenge Miocic was only the third superfight in UFC history between reigning champions of two weight classes.
Welterweight champ Georges St- Pierre defended his belt in 2009 by battering lightweight champ BJ Penn, and McGregor accomplished his feat less than two years ago in his most recent MMA fight.
After beating Miocic, Cormier engaged in a bit of clearly planned theatrics with Brock Lesnar, the professional wrestler and former UFC heavyweight champion.
Cormier called Lesnar into the cage, and Lesnar shoved Cormier in the chest while the fighters traded insults to hype a potential future bout.
"Push me now, (and) you're going to sleep later," Cormier yelled at Lesnar. "Your days are in the past. In the stone age."
Lesnar is widely expected to return to the UFC later this year or early next year.
Earlier, heavyweight Derrick Lewis earned a lifeless decision over formerly fearsome prospect Francis Ngannou in the co-main event at the UFC's biggest show of the summer.
Welterweight Mike Perry also earned a copiously bloody split-decision victory over Paul Felder.