It is the second recent case at the club after a confirmed positive test at the end of December, and no one else at the club is affected.
The Perth side are preparing to face Dundee United in the Scottish Premiership on Saturday.
St Johnstone released a statement on their official website which read: "The football club can today confirm that a member of our playing staff has tested positive for Covid-19 and is now self-isolating at home.
"We wish him well as the health and wellbeing of our players and staff will always be our priority.
"This is an isolated case and the player has not been in close contact with any member of staff at the football club.
"We have contacted all of the relevant authorities to make them aware of the situation and will keep them all updated.
"Our Scottish Premiership match at Dundee United this Saturday is unaffected."