Faultless Argentinian fly-half Benjamin Urdapilleta kicked 19 points to guide Castres to a convincing 29-13 victory over Montpellier in the Top 14 final on Saturday.
Castres sprang out to lead 19-6 at the break, Urdapilleta on target with four penalties before converting Julien Dumora's try, misfiring Springbok Ruan Pienaar hitting two penalties for Montpellier, league table toppers at the end of regulation season.
Montpellier hit back with a deserved penalty try in the second period, Urdapilleta knocking over his fifth penalty before Steve Mafi crashed over for his team's second try with five minutes to play.
Stand-out Castres centre Thomas Combezou admitted his team had expected a "tight physical war of attrition" as they tied up a fifth French league title, the last coming in 2013.
"We wanted to win and we did it!" he said. "It's the magic of rugby and knock-out rugby. It's exceptional."
Castres captain Mathieu Babillot said his team had "gone looking for it".
"We knew Montpellier were a team with few weak points and that we had to be present in every part of the game," he said.
Montpellier coach Vern Cotter, whose side are yet to win a French title having also finished runners-up in 2011, added: "We made too many mistakes.
Six months of work and today we just didn't turn up.
"We weren't ourselves. Castres played a good match, you can't take anything away from them. We're disappointed, we couldn't express ourselves."
Castres coach Christophe Urios had warned his side, which finished in the sixth and final play-off spot, were "laid back but ambitious" and so it proved against a star-studded Montpellier side for whom Pienaar and All Black fly-half Aaron Cruden were unable to dictate the flow of play.
Pienaar missed a fourth-minute penalty, but Urdapilleta made no such mistake a couple of minutes later to open the scoring at a packed Stade de France.
Montpellier were eventually on target in the 12th minute, Bok centre Frans Steyn booting a three-pointer.
Urdapilleta, capped 10 times by Argentina, restored Castres' lead with his second and third penalties in quick succession in a close first quarter of a physical match.
Steyn went wide with a penalty effort from his own half, but Montpellier -- with 10 of the starting XV foreigners (6 South Africans, and one each from Australia, Fiji, Georgia and New Zealand) -- were made to pay for a further indiscretion as the 32-year-old Urdapilleta booted his fourth penalty.
Pienaar pulled one back to make it 12-6, but Castres, with a heroic defence against a flat-footed Montpellier, delivered a hammer blow just before the break.
Boxed into their own 22-metre area, Montpellier botched the line-out with an untidy slapdown that went dead to gift Castres a 5m scrum.
Scrum-half Rory Kockott consequently worked his forwards, Urdapilleta a decoy in the box for a possible drop-goal.
But suddenly Kockott moved the ball wide, Combezou straightened before feeding Dumora, who feigned a pass to scythe past a leaden-footed Fulgence Ouedraogo for a try, Urdapilleta hitting the extras.
Montpellier, undoubtedly fired up by a rocket of a half-time talk from Cotter -- who previously lost three Top 14 finals with Clermont (and won one), took the game by the scruff of the neck at the start of the second period.
Castres held out as long as they could before first lock Loic Jacquet was yellow carded and then a penalty try was awarded as Montpellier took advantage of their numerical advantage in the subsequent scrum.
Remarkably, Castres then went on to retain possession through a long sequence of plays, running down the remainder of the 10 minutes Jacquet was in the sin bin. It ended with a high tackle on winger David Smith, Urdapilleta kicking his fifth penalty.
The off-target Pienaar put another penalty wide after a charge by Alexandre Dumoulin, spurning another chance to go for the corner.
That backfired, however, as Castres masterfully worked the line-out maul into touch.
Moving back upfield, Castres' Tongan replacement Mafi crashed over from close range to drive the nail into Cotter's hopes, Urdapilleta tellingly having the final word with a successful conversion to cap an outstanding individual performance.
Tries: Dumora, Mafi
Cons: Urdapilleta 2
Pens: Urdapilleta 5
Pens: Steyn, Pienaar