A closely fought tie came to life in the final 18 minutes as the Perth side struck twice in two minutes through Chris Kane, who flicked home a cross from substitute Glenn Middleton, who then doubled the lead with a stunning 25-yard free-kick.
An 85th minute header from Conor McCarthy pulled one back for the Paisley side and set up a tense finish but Callum Davidson’s men held on to reach their second final of the season, having lifted the Betfred Cup in February.
St Johnstone will now meet Hibs in the final on Saturday, May 22 after the Edinburgh side overcame Dundee United in Saturday’s other semi-final.
Here is how the two sets of Saints players rated ...
21. Craig Bryson - 7
Defied his age to move around the park, always offering himself as an option and advancing play. Photo: Craig Williamson - SNS Group
22. Callum Booth - 8
Particularly impressive in the second period where he constantly drove forward from defence. Photo: Alan Harvey - SNS Group
23. Chris Kane - 8
A terrific finish for the opener and did a lot of selfless work in attack. Photo: SNS Group Ross Parker
24. Guy Melamed - 6
The most disappointing of the St Johnstone starters. Struggled to impact the game. Photo: Mark Scates - SNS Group