The 25-year-old spent last season on loan at West Midlands rivals Aston Villa, reaching the play-off final before losing to Fulham at Wembley Stadium.
Johnstone has signed a four-year contract at the Hawthorns.
The 6’4″ goalkeeper never made a single appearance for Manchester United and had spent several seasons away from the club on loan before making a permanent move today. Johnstone’s previous loan spells include Scunthorpe United, Doncaster Rovers and Preston North End before joining Aston Villa last season.
With Ben Foster expected to leave the Baggies this summer and with Boaz Myhill being released, Sam Johnstone can be expected to become Darren Moore’s first choice goalkeeper for the 2018-19 season.
Sam Johnstone has been replaced at Manchester United by experienced goalkeeper Lee Grant.
West Bromwich Albion start their 2018-19 Championship campaign at home to Bolton Wanderers on Saturday 4 August.