Pierre Aguillon's second-half sending-off proved crucial as Toulon came from behind to beat La Rochelle 18-15 and reach the Top 14 final with an 81st-minute drop-goal from Anthony Belleau.
La Rochelle finished first in the regular-season standings and appeared in command of Friday's semi-final, played in front of a record crowd for a last-four clash, at the Stade Velodrome.
They led a contest in which the points all came from the boot 15-6, only for the game to turn dramatically in Toulon's favour in the 52nd minute.
Aguillon was shown a red card after he was adjudged to have committed a dangerous tackle on Toulon's James O'Connor.
O'Connor landed on his face having been lifted off the ground and turned in mid-air and Aguillon's subsequent dismissal enabled Toulon - beaten in last season's final - to mount a comeback and draw level at 15-15.
Zack Holmes missed two difficult kicks that would have re-established a lead for La Rochelle and his profligacy was punished as, after a penalty to Toulon gave them a line-out 10 metres out, Belleau knocked over a simple drop-goal to complete the turnaround.
Brock James had the edge in the kicking battle with Leigh Halfpenny at the interval, his boot giving La Rochelle a slender 9-6 lead after 40 minutes.
A simple penalty in front of the post and an effort from James from 45 metres out made it a nine-point gap six minutes into the second half.
But the momentum shifted drastically after the TMO decided Aguillon's tackle was dangerous enough to warrant a red card, Halfpenny immediately converting a three-pointer from 40 metres in the wake of the sending off.
His fourth penalty of the contest cut it to 15-12 and the Welshman, who will not be able to play in the final due to his commitments with the British and Irish Lions, levelled matters 11 minutes from time.
Having replaced James, Holmes twice failed with kicks from 50 metres out, but La Rochelle still appeared to have done enough to force extra time.
Yet there was to be a final late twist as Toulon surged towards the posts from their late line-out, Belleau keeping his composure to convert a routine drop-goal from in front and book their place in the Stade de France showpiece against either Clermont Auvergne or Racing 92.