The 21-year-old centre-back had previously played for the Saints during the second half of the 2024/25 campaign.
He then went out on loan to Scottish Premiership side Hibernian, where he made just two appearances under manager David Gray.
But now, he will return to Perth as St Johnstone prepare to return to the Scottish Premiership after securing the Scottish Championship title last season under club boss Simo Valakari.
In an official club statement, Mitchell said: “I am extremely excited. I have been at the club before, and I thoroughly enjoyed my time when I was here.
“I was gutted last time to not be able to see the season out due to injury, so I almost feel I have unfinished business here. I want to hit the ground running.
“I know my body and my game better. I am really positive and optimistic about the season ahead.”