The pair will both miss out on the trip to Hampden, while two other players are being forced to isolate as well due to being deemed as close contacts.
It means that manager Callum Davidson’s preparations have been thrown into disarray as the Saints look to complete a historic cup double after claiming the Betfred Cup already this season.
A statement from St Johnstone read: “The Football Club can today confirm that two members of our playing staff have tested positive for Covid-19.
“They will both be missing from the Scottish Cup semi-final on Sunday against St Mirren.
“Thankfully, after the latest round of testing, all results were negative.
“However, as a result of the positive tests, two other players have been forced to self-isolate due to Government guidelines. The two players in question tested negative.
“We feel our staff adhered to the guidelines and this has been largely outwith our control.
“It's very unfortunate for four of our players to miss out on the important game on Sunday.
“However, we wish all of them well and their health and wellbeing will always be our priority.
“We will keep our supporters updated if we need to share further information.
“Callum and the players will now focus on Sunday's game.”