The Uruguayan veteran, who's Paris Saint-Germain's all-time leading scorer, was available on a free transfer and was linked with moves to Juventus, Atletico Madrid, Real Madrid, and Benfica earlier this summer.
Cavani signed a one-year contract at Old Trafford with an option to extend the deal for an additional year. The striker's agent was reportedly requesting a payment of €10 million before signing off on Cavani's transfer.
"I have played in front of some of the most passionate supporters in football during my career and I know that it will be the same in Manchester," he said. "I cannot wait to experience the Old Trafford atmosphere, when it is safe for the fans to return."
The 33-year-old will challenge Marcus Rashford, Anthony Martial, Mason Greenwood, and Odion Ighalo for minutes in Ole Gunnar Solskjaer's frontline. He officially left PSG at the end of June after falling behind Kylian Mbappe, Neymar, Mauro Icardi, and others in the stacked striking contingent at the Parc des Princes.
Manchester United completed the signings of left-back Alex Telles and 18-year-old winger Amad Diallo Traore earlier on deadline day. The Red Devils had only picked up Donny van de Beek from Ajax this summer before the 11th-hour acquisitions of Cavani, Telles, and Traore.