Willock spent the first half of this season on loan at FC Utrecht, and the 21-year-old made three senior appearances for the Dutch side before returning to Manchester.
Willock will now spend the remainder of the 2017-18 campaign on loan in Scotland.
"Manchester United youngster Matty Willock has returned from his loan spell at FC Utrecht, to embark on another temporary spell at Scottish Premiership side St Johnstone," read a statement on Man United's official website.
Meanwhile, St Johnstone boss Tommy Wright said in a statement: "I'm really pleased to get Matty who has been on loan at FC Utrecht. He is a midfielder who possesses a lot of qualities, very comfortable on the ball, physically as well.
"He's comfortable playing as a holding midfielder or attacking midfielder. When you get someone who's gone through the academy at Arsenal and Man United he's going to provide real quality in the midfield for us. He will go straight into the squad for Saturday's game at Hearts."
Willock is yet to make his first-team bow for United, but was named on the bench for their league games against West Bromwich Albion and Crystal Palace in the latter stages of last season.