Youssef El-Arabi scored in the last minute of extra-time as Olympiakos knocked Arsenal out of the Europa League on Thursday on a night when four former European champions were eliminated.
Arsenal had won the first leg of the round of 32 tie 1-0 in Greece.
Defender Pape Abou Cisse cancelled out that advantage when he headed in a corner after 53 minutes.
Arsenal could not break through and were forced to extra time.
Giorgos Masouras hit the bar for Olympiakos in the 112th minute and a minute later Pierre-Emerick Aubameyang struck for Arsenal, putting them back in front with a spectacular flying kick.
However, in the final minute of extra time, Arsenal cleared an Olympiakos corner but were still disorganised when the ball was swung back into the area.
The cross found El Arabi free in front of goal for an unchallenged shot who slotted the ball home amid ecstatic celebrations from the thousands of travelling Greek fans in the stands.