A rematch between Anthony Joshua and his conqueror Andy Ruiz Jr will take place in November or December says Joshua's promoter, Eddie Hearn.
American-born Mexican Ruiz produced one of the biggest shocks in heavyweight boxing history to take Joshua's IBF, WBO and WBA world heavyweight titles in New York on Saturday.
A rematch clause was part of the contract for the fight.
Hearn said the clause had been triggered on Tuesday after talks.
"After meetings with AJ, Rob Mc[Cracken - Joshua's trainer] and the management team in NY, we have today triggered the contracted rematch clause with Andy Ruiz Jr," Hearn said on Twitter.
"The fight will take place in Nov/Dec at a venue to be confirmed shortly."
British fighter Joshua, 29, was floored four times as Ruiz defied his underdog status to win in seven rounds at Madison Square Garden. The London 2012 Olympic champion subsequently had a concussion test after the first defeat of his 23-fight career.
Ruiz, also 29, took the fight at short notice after Joshua's original opponent Jarrell Miller failed drug tests.
The new champion, who was born in the United States but fights under the Mexican flag, has previously said he will take a rematch at any time.
In a post-fight news conference, Joshua was asked about potential fights with WBC world champion Deontay Wilder and Tyson Fury.
"I still want to compete with those men but a rematch with Andy Ruiz Jr is where we are at," he said. "I'm still a champ and I will rectify it to get the belts back."
At the same time, Hearn said a rematch is a "must win" because Joshua's options will be "nothing after that".