The 25-year-old has signed a four-year deal with the Baggies to become their first summer recruit.
"I'm happy to be here. I've spoken to a lot of people about the club and heard nothing but good things," Johnstone told West Brom's official website.
"It's good to get through the door. I've played Championship games and won a promotion from League One with Preston but always seemed to end back at Manchester United or going somewhere else.
"Now it's time to settle down and take my game up to the next level which I believe I can do here."
Johnstone, who will replace veteran Ben Foster between the West Brom sticks, spent last season on loan with Aston Villa.