The 22-year-old joined his hometown club in January 2015 and spent the second half of the last season on-loan at Deepdale. During his time at the club, the stopper helped his side to achieve promotion to the Championship.
In the League One play-off final, Johnstone was in between the post as Preston moved to the second tier of English football after defeating Swindon Town 4-0 at Wembley.
“Manchester United goalkeeper Sam Johnstone is joining Preston North End on loan for the second time in his fledgling career, after helping his hometown team earn promotion to the Championship last season,” a statement read on United’s official website.