The 25-year-old striker has been released early from his two-year contract at Home Park having failed to establish himself in Bobby Williamsons first team squad since moving to Argyle a year ago.
"St Johnstone approached us and spoke to Steven, and they can offer him a longer-term contract than we were prepared to," Williamson told the clubs official website.
"I didn't want to stand in the lad's way so he will be going back to Scotland to further his career.