Darlow joins the Red Devils as a free agent from Leeds.
The 32-year-old has now signed a contract until June 2028, with the option to extend it for another year.
Darlow’s arrival comes days after United brought in midfielder Andrey Santos from Chelsea for £50 million.
United have also also activated Youri Tielemans’ £35m release clause at Aston Villa and he has undergone his medicals.
Darlow is expected to provide back-up and competition for first-choice goalkeeper, Senne Lammens.
Altay Bayindir, who has been the deputy to Lammens and Andre Onana, is now set to leave Old Trafford.
Onana has already agreed on a new loan deal at Trabzonspor, to play at the Turkish club for a second season.