Second-half goals from Reece McAlear and Ruari Paton sealed St Johnstone's instant return to the William Hill Premiership at East End Park.
A 2-0 victory over Dunfermline moved Saints seven points ahead of Partick Thistle with two games left in the Championship season.
Partick Thistle's draw with Airdrie on Saturday had handed Simo Valakari's side two chances to claim the title before the Jags next play and they seized their first opportunity.
The Perth side have led the table from the opening day, when they beat Thistle 5-1, although the Maryhill side were level on points at one stage and both teams are on 12-match unbeaten runs in the league.
St Johnstone were taking on a side who played 120 minutes at Hampden on Saturday before their penalty shootout triumph over Falkirk in the Scottish Cup semi-finals and who were playing their third consecutive midweek game.
Dunfermline manager Neil Lennon made five changes to his starting line-up.
Saints had the better of the first half and Josh McPake was yet again the danger man for the visitors. The winger hit the underside of the bar from 15 yards and delivered several teasing balls which Paton could not take advantage of.
Sam Stanton also came close from long range and inadvertently blocked a goal-bound effort from team-mate Morgan Boyes.
The breakthrough came in the 56th minute after the hosts failed to clear their lines. Paton teed up McAlear 22 yards out and the former Livingston midfielder's first-time shot took a major deflection off the boot of Charlie Gilmour and deceived goalkeeper Aston Oxborough.
The Perth side doubled their lead in the 72nd minute when Paton drifted in between the Dunfermline centre-backs and guided a header into the bottom corner from 12 yards following Liam Smith's cross.
Dunfermline were left with a five-point lead over fifth-placed Raith Rovers, but they could wrap up a play-off place before the weekend with their Fife rivals travelling to McDiarmid Park on Friday.
St Johnstone captain Jason Holt was proud and relieved to seal their return to the top flight after bouncing back from a difficult spell in his career.
Holt suffered successive relegations with Livingston and Saints but he led the Perth side back to the William Hill Premiership as a 2-0 win at Dunfermline sealed the Championship title.
After second-half goals from Reece McAlear and Ruari Paton sparked a party at East End Park, Holt told BBC Scotland: "It's just relief to get over the line. Testament to the staff and players because we set the standard from day one and we stayed there.
"Fair play to other teams putting pressure on us but we knew we were the best team in the league.
"And to get it done at the first opportunity, it's just great to get promoted back to the top flight."
Holt praised manager Simo Valakari for leading Saints to promotion in his first full season in charge.
"What you see is what you get, really, so much positivity, so much enthusiasm," the former Hearts and Rangers midfielder said.
"The biggest compliment I can give him, as players we go on the pitch feeling free. That's all you can ask as a player. Make mistakes, it's fine, don't worry about it. That's given us the platform to be successful this year.
"He's been different class since he came to the club.
"Last year at times was super difficult. There aren't many players in the dressing room who were here last year, I'm one of probably two or three. Really difficult at times. Takes over your life.
"But to bounce back at the first time of asking, I'm honestly so proud of the lads."
