The 21-year-old, who is a member of Milngavie, started the final round at Infinitum Golf in Tarragona two shots behind Welsh woman Darcey Harry.
But, after signing for a closing one-over 72, McClymont joined 18-year-old Harry at the top of the leaderboard on five-over 218.
McClymont, a sports studies student, forced the play-off with a closing birdie-4 then repeated the feat at the first extra hole.
“I knew I had to go for it at the last and was pleased to see the ball land on the green,” said McClymont, who had already won events in Portugal and Ireland.
“In the play-off, I was even closer to the green and hit a 4-iron, so that hole was crucial for me today.
“It’s a bit special to think I’ve won the last three (Tour Series events). It gives me a lot of confidence for the rest of the season.
“I’m back in Spain in two weeks for the European Nations Cup at Sotogrande, and how I played this week will help me with that experience. It will also help me for the rest of the season.”
McClymont was praised by Dean Robertson, the University of Stirling’s performance director.
He said: “Lorna’s been working hard on her game and this win just proves it. She managed her game well today. Those two shots at 18 were clutch. I’m proud of her.”
Louise Duncan, the Women’s Amateur champion from West Kilbride, finished seventh on 12-over, having opened with an 80 before adding scores of 74-71.
Megan Ashley (Blairgowrie) tied for eighth on 15-over, with Auchterarder’s Penelope Brown finishing 11th on 23-over.
The men’s event in Spain was also decided in a play-off, with Ireland’s Eoin Sullivan (Trinity College) denying a Stirling double as he beat Ben Brookes in similar fashion.
Roddy McAuley (Fereneze) tied for 13th on 10-over, one ahead of Lachlan Reynolds (Rothesay).
The top players will qualify for The R&A Student Tour Series Final, which takes place over Dumbarnie Links and the Castle Course and Jubilee Course in St Andrews on 4-6 April.