Inter Milan goalkeeper Andre Onana. AP
Onana, 27, played a major role in Inter's run to the Champions League final and previously worked with United manager Erik ten Hag at Ajax.
The Cameroonian will reportedly sign a five-year deal at Old Trafford to become the successor to David De Gea as the club's number one goalkeeper.
United will pay an initial 51 million euros with a further four million euros due in performance-related add-ons.
De Gea left the Red Devils earlier this month after 12 years at United.
The Spaniard struggled with Ten Hag's preference to play out from the back and Onana appears a far more natural fit for the Dutch coach's philosophy.
Onana joined Barcelona as a 14-year-old after being spotted as Samuel Eto'o's academy in his homeland.
After failing to make the first team at the Camp Nou, Onana joined Ajax in 2015 and won three Eredivisie titles under Ten Hag in the Netherlands.
However, his career was temporarily rocked in 2021 by a doping ban for taking a banned diuretic.
Onana claimed to have accidentally taken a drug prescribed to his wife and had a one-year ban reduced to nine months on appeal at the Court of Arbitration for Sport.
He joined Inter on a free transfer just over 12 months ago when his contract at Ajax expired.
Onana made only 24 Serie A appearances as he shared goalkeeping duties with Samir Handanovic in the Inter captain's final year at the club.
But he was an ever-present in the Champions League, keeping eight clean sheets in 13 games as Inter reached the final for the first time in 13 years.
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