McClean, who was recently released by Nottingham Forest, was on trial with the Perth outfit in the last few weeks of the 2016-17 campaign.
And the midfielder has now signed a one year deal at McDiarmid Park.
Manager Tommy Wright told the club's website: "I was very impressed with Kyle in his trial and he will go straight into the first team squad."
McClean has been included in Northern Ireland's squad for their opening Euro Under-21 qualifier in Estonia on Thursday.
St Johnstone finished fourth in the Premiership in 2016-17 and go into the Europa League qualifiers later this month.
Find all the latest football transfers on our dedicated page.
St Johnstone have made 18-year-old Northern Irish midfielder Kyle McClean their first signing of the summer.