Justin Gaethje beat Ilia Topuria after the fourth round of their UFC Freedom 250 bout to secure a huge upset win over the lightweight champion outside the White House.
The American had bloodied his Spanish-Georgian opponent with a left-right combination to the face in the third round, leading to an extended break after the round in which doctors seemed to suggest the fight should end.
Topuria's corner then called for the fight to be stopped after the fourth round.
After securing the UFC lightweight title Gaethje was greeted by President Donald Trump, who was hosting the event to celebrate his 80th birthday and the nation's 250th anniversary, in the Octagon.
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In other fights, Ciryl Gane knocked out Alex Pereira to win the interim UFC heavyweight title.
French fighter Gane overcame the Brazilian after the referee stopped the action in the second round at 1:27, following a left to the chin.
Mr Trump held the event on the White House South Lawn with seven fights in total.
Sean O'Malley secured a knockout of Canadian Aiemann Zahabi in their bantamweight bout, saluting troops afterwards, while Diego Lopes knocked out Steve Garcia in a featherweight contest.
Mauricio Ruffy knocked out Michael Chandler in a lightweight contest, while Josh Hokit saw off Derrick Lewis in the second round of a heavyweight fight.
Earlier in the night, Bo Nickal knocked out Kyle Daukaus in the first round of their middleweight fight.
Fighters warmed up in different storied rooms of the capital landmark, each of them passing through the Oval Office on their way to the Octagon.
The unusual event also came just hours after Mr Trump confirmed a deal to end the war in Iran and allow oil traffic to begin again through the Strait of Hormuz.