Mont-de-Marsan triumphed 29-20 over Pau in Sunday's promotion play-off to join Grenoble in the Top 14 next season.
Grenoble earned automatic qualification to the top flight of French rugby by finishing 18 points clear at the top of the ProD2 standings.
Pau finished second in the standings but had to face Stade Montois, who finished third, three points further back, in the promotion final in Bordeaux.
The pressure of the big occasion proved too much for Pau as Mont-de-Marsan returned to the Top 14 after being relegated in 2009.
Mont-de-Marsan led 16-10 at half time. Wing Martin Jagr added his second try of the game, from 70 metres out, shortly after the restart to give le Montois a healthy lead before scrum-half Benat Arrayet's two late penalties secured victory.