La Rochelle were promoted to the Top 14 after defeating Agen 31-22 in the Pro D2 play-off final in Bordeaux on Sunday.
La Rochelle dominated the opening period as they took a 21-3 lead into the half-time break courtesy of a try from wing Damien Cler and a penalty try that was awarded just before the interval.
Sireli Bobo touched down early in the second half to put them further in front but Agen staged a rousing comeback with three tries of their own to claw to within nine points and ensure a nervy finish.
However, La Rochelle held on, avenging a pair of regular-season losses to Agen, to secure a return to the top tier of French rugby after a three-year absence.
"It's a happy, marvellous ending," said La Rochelle president Vincent Merling.
"We've been working tirelessly since 2011 to return to the Top 14 in 2015. We dominated the first half but then got frightened, Agen came back with an extraordinary speed.
"But we've made it. I've less fear now of going up to the Top 14 than the final moments of that match."
They will join D2 champions Lyon in next season's Top 14, taking the places of relegated duo Perpignan and Biarritz.
La Rochelle have invested heavily in a return to the top flight and with a stadium capacity set to increase to 15,000 and a budget of 16.5 million euros have given themselves the means to compete against elite teams such as Toulon who retained their Heineken Cup title on Saturday.
"I still can't believe that we're going to be playing at Toulouse, Clermont," said an emotional coach Fabrice Ribeyrolles.
"I know that we're going to have a lot of work to do in the build-up to the new season. Next year is going to be very complicated. The hardest is yet to come.
"I'm already worried. I saw Toulon yesterday night... Oh la la, it's going to be something else. But we've made it. That's the main thing."