Munster was dumped out of the Heineken Cup at Stade Anoeta when losing 7-18 as Dimitri Yachvili booted Biarritz into an all-French final in Paris.
Two-time champion Munster was firmly in control after a slick try from Keith Earls, converted by Ronan O'Gara, helped it establish a 7-3 interval lead.
But Biarritz, which included English trio Iain Balshaw, Magnus Lund and Ayoola Erinle in its starting line-up, ground it into submission in the second half.
It never seriously threatened the whitewash but with Munster falling foul of referee David Pearson's whistle and Yachvili kicking the penalties - six from six attempts - it crept in front.
Starved of possession and struggling to escape its own half, the Irish province faded from contention to ensure its search for a first win in San Sebastian continues.
Biarritz had endured a poor season in the French Top 14 with its seventh-place finish not enough to qualify for next season's Heineken Cup, making Sunday's victory all the more crucial.
It was a scrappy match, marred by a succession of handling errors that was not entirely explained by the heavy rain that preceded the match.
The repeat of the 2006 final, which Munster edged 23-19 to win its first Heineken Cup title, failed to live up to expectation but remained tense until Biarritz pulled clear late on.
The French club started slowly, however, with Munster enjoying early success at the scrum, winning a penalty at one and disrupting its opponents on a second.
Combined with repeatedly poor hands from the home side, this enabled it to control the first 10 minutes with full-back Paul Warwick missing a dropped goal attempt.
Karmichael Hunt and Balshaw kicked out on the full while the string of dropped passes also began to afflict Munster as it became Denis Hurley's turn to spill possession.
Springbok centre Jean de Villiers was flattened onto the turf by Benoit August but Munster renewed their offensive and could have surged ahead in the 19th minute.
Using Hurley outside him to create doubt in the Biarritz defence, hooker Jerry Flannery burst into space before spotting O'Gara on his shoulder but the pass failed to stick.
Flannery was also involved as Munster exploited gaps around the breakdown once again on the half-hour mark and this time the move finished with a try.
Donnacha O'Callaghan marauded into space, the superb Quinlan kept the move going before Flannery distributed wide where David Wallace supplied the scoring pass to Earls.
The conversion was added by O'Gara and Biarritz responded with an attack of its own which finished when August was shoved into touch a metre out.
The Top 14 side eventually troubled the scoreboard in the 39th minute with a penalty from Yachvili, who added a second shortly into the second half.
De Villiers spotted a slight opening and stepped on the gas with Marcus Horan continuing the move until Flannery knocked on.
Smart kicking from Hunt kept Munster pinned back until O'Gara missed a long-range penalty, which preceded a fresh assault from Biarritz.
The tide was turning but the French side made a mess of a fine chance when Arnaud Mignardi failed to spot an overlap, giving Balshaw little chance of reaching the line.
It took the lead for the first time in the 65th minute when Tomas O'Leary prevented the ball from being released and Yachvili slotted the three points.
Biarritz was expertly tightening the screw but was lucky to win Yachvili's fourth penalty after Tony Buckley was pinged for picking up prop Campbell Johnstone and tossing him to one side.
Six more points from Yachvili followed as Munster's discipline crumbled late on to wave Biarritz into the final on May 22.