Castres will face Toulon in the Top 14 Final next week after claiming a shock 25-9 victory over Clermont in their semi-final at Stade de la Beaujoire in Nantes on Saturday.
Against all expectations, Castres led from start to finish to reach their first final since 1993.
Clermont were a mere shadow of their normal selves and suffered a nightmare afternoon, particularly in the scrum where Castres dominated.
South African scrum-half Rory Kockott kicked 20 points and centre Romain Cabannes raced 70 yards to touch down the only try of the game 16 minutes from time after intercepting Mike Delany, who kicked three penalties for Clermont.
The underdogs took a 12-3 lead into the interval thanks to a very well organised defence, a dominant scrum and four penalties from the boot of Kockott.
While Castres were playing as a cohesive unit, Clermont looked like they were yet to fully digest the heartbreak of their defeat in the European Cup Final a week earlier.
Clermont were unable to impose their usual high-tempo on the game as les Castrais flooded the breakdown and effectively disrupted Clermont's attacks.
A measure of Clermont's troubles was evident from the first quarter-of-an-hour as scrum-half Morgan Parra, usually so reliable with the boot, missed two penalties.
That allowed Castres to take a 6-0 lead as Kockott proved unerring himself from the tee and he had notched four successful kicks by half-time as Delany took over from Parra and kept Clermont in touch, although they trailed by nine points at the interval.
The addition of Thomas Domingo off the bench stabilised the Clermont front-row but the points kept flowing for Castres, with Kockott and Mike Delany exchanging three pointers to leave the scores at 18-9 on the hour mark.
Castres landed the killer blow in the 63rd minute. Clermont really needed a try to turn the momentum in their favour but Delany's overly ambitious pass to Wesley Fofana was cut out by Cabannes and with that Clermont's hopes went up in smoke.
Clermont dominated the closing minutes but were unable to crack the CO defensive wall.
Clermont had finished top of the table during the regular season, scoring the most points and conceding the fewest. Having beaten Castres comfortably at home earlier in the campaign, although they were narrowly beaten away, they were expected to storm to victory. But they failed to produce their trademark swashbuckling rugby as they limped out of the competition with barely a whimper.
The result means coaching pair Laurent Labit and Laurent Travers may yet bow out in ultimate style before leaving to take over at big-spending Racing-Metro next season.
Castres have not won the French title for 20 years and are looking for a fourth crown. Toulon likewise are going in search of a fourth title and their last win was the year before Castres' in 1992.
Pens: Delany 3
Yellow card: Bardy
Pens: Kockott 6
Yellow card: Claassen
Clermont: 15 Lee Byrne, 14 Sitiveni Sivivatu, 13 Aurélien Rougerie (c), 12 Wesley Fofana, 11 Naipolioni Nalaga, 10 Mike Delany, 9 Morgan Parra, 8 Damien Chouly, 7 Julien Bardy, 6 Julien Bonnaire, 5 Loïc Jacquet, 4 Julien Pierre, 3 Davit Zirakashvili, 2 Benjamin Kayser, 1 Raphael Chaume.
Replacements:16 Ti'i Paulo, 17 Thomas Domingo, 18 Nathan Hines, 19 Alexandre Lapandry, 20 Ludovic Radosavljevic, 21 Brock James, 22 Benson Stanley, 23 Daniel Kotze.
Castres: 15 Brice Dulin, 14 Romain Martial, 13 Romain Cabannes, 12 Seremaia Baï, 11 Marc Andreu, 10 Remi Talès, 9 Rory Kockott, 8 Antonie Claassen, 7 Yannick Caballero, 6 Ibrahim Diarra, 5 Rodrigo Capo Ortega, 4 Christophe Samson, 3 Karena Wihongi, 2 Brice Mach, 1 Saimone Taumoepeau.
Replacements: 16 Mathieu Bonello, 17 Yannick Forestier, 18 Joe Tekori, 19 Janie Bornman, 20 Romain Teulet, 21 Daniel Kirkpatrick, 22 Paul Bonnefond, 23 Mihaïta Lazar.
Venue: Stade de la Beaujoire, Nantes
Referee: Christophe Berdos
Assistant referees: Patrick Péchambert and Sébastien Minery