After earning a stoppage-time win in France a fortnight ago, Olympiakos again left it late before snatching victory with two goals in the last 10 minutes from Leandro Greco and Kostas Mitroglou.
Those strikes came after Younes Belhanda's second-half penalty for Montpelier had cancelled out Paulo Machado's early opener for the Greek side.
Olympiakos had lost their opening two matches in Group B but Tuesday night's result lifts them onto six points, two points behind leaders Schalke and one adrift of second-placed Arsenal, while Montpellier remain on one point and all but out of contention.
Olympiakos had fallen behind in each of their opening three games in Group B but needed just four minutes to take the lead when Machado slotted into an empty net after Mitroglou's close-range effort had rebounded back off the post.
Montpellier could not get back on terms before the break but they came close again in the second half when substitute Jonathan Tinhan, just moments after replacing Anthony Mounier in the 62nd minute, fired in an effort that was saved by the feet of Roy Carroll.
Carroll was called into action again seconds later after Joris Marveaux was found unmarked in the area but fortunately for the Northern Irish veteran the midfielder's header was straight at him.
An equaliser looked on the cards, though, and it duly arrived in the 65th minute when the French side won a penalty.
Dimitrios Siovas pushed Camara in the area to concede the spot-kick and Belhanda made no mistake from 12 yards.
For a while that looked set to earn Montpellier a point, but Olympiakos had other ideas and claimed the victory following a late flourish.
Djamel Abdoun saw a long-range effort turned round the post by Jourdren in the 77th minute but three minutes later Greco found the back of the net following a corner.
Two minutes after that, the game was sealed when Mitroglou slotted in Jose Holebas' cross to make it 3-1.