The Irish province made a dream start and were 17-0 up after 11 minutes thanks to a try from Jimmy O'Brien and two for Dan Sheehan but the defending champions managed to cut the halftime deficit to nine thanks to tries from centres Jonathan Danty and UJ Seuteni.
The ferocious first 40-minute pace turned into a game of high tension as soon as the second half began and the pendulum swung decisively La Rochelle's way nine minutes from time when Antoine Hastoy converted a Georges Henri Colombe try.
Leinster went straight back up the other end but their late burst ended with a Michael Ala'alatoa red card as the French side, who won their maiden title last year against the four-time winners, joined Saracens, Toulon, Leinster and Leicester in successfully defending their title.
