Pacquiao, however, had said that Horn was one of many possible opponents.
"Negotiations between team Pacquiao and team Khan have come to terms for the April 23 bout as this is what the fans wanted," Pacquiao tweeted.
The 30-year-old Khan, who won silver as a lightweight at the Athens Olympics, has not fought since last May when he moved up two weight classes to fight middleweight Canelo Alvarez.
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The much bigger Mexican proved too powerful and the Briton was knocked out in the sixth round and he is likely to drop back to welterweight to fight Pacquiao, who holds the World Boxing Organisation title at the weight.
"My team and I have agreed terms with Manny Pacquiao and his team for a super fight," Khan said on his Twitter page.
Pacquiao won the WBO welterweight title last November against American Jessie Vargas in Las Vegas with an unanimous decision.
No venue for the fight was announced.
(Reporting by Greg Stutchbury; Editing by Nick Mulvenney)
