The 35-year-old arrives after deciding against extending his stay at Leeds United, despite being offered fresh terms when his contract expired earlier this summer. Leeds confirmed his departure without mentioning his move to their Premier League rivals.
"Leeds United can confirm Karl Darlow has left the club upon the expiration of his contract to pursue an opportunity elsewhere," Leeds said in a statement.
"Despite the offer of a new deal to remain in West Yorkshire, the goalkeeper wished to explore a different challenge and leaves the Whites with the best wishes of everyone at Elland Road."
Darlow finished the previous campaign as Daniel Farke's first-choice goalkeeper and brings significant experience to Old Trafford, having made 279 club appearances and collected 15 senior caps for Wales.
The veteran shot-stopper is expected to serve as deputy to Senne Lammens, who is currently on a three-week break following Belgium's FIFA World Cup campaign.
"I am extremely proud to sign for Manchester United," Darlow said.
"I'm joining an excellent group of goalkeepers and I'm really looking forward to all pushing each other to ensure that we maintain the highest standards, which this club demands."
With Lammens unavailable, Darlow is likely to feature during Manchester United's pre-season programme, which gets underway against Wrexham in Helsinki on 18 July.
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Michael Carrick believes Andrey Santos showed plenty of promise on his Manchester United debut despite the club's 1-0 pre-season defeat to Wrexham in Helsinki on Saturday.
Manchester United fell to a 1-0 defeat to Wrexham in a preseason friendly in Helsinki on Saturday as new signing Andrey Santos made his debut.
