19-year-old Johnstone is a product of the United academy and has represented England at Under 16, 17 and 19 levels.
Sam has yet to make his full debut for the Red Devils but along with his England experience has spent time on loan with Oldham Athletic and Scunthorpe United, his acquisition is a major boost to the Saddlers following this weeks departures of former first choice David Grof and loanee Aaron McCarey.
Sam goes straight into the squad for Saturday's trip to Hartlepool United.