By the time their game had started, Panathinaikos, the only team that could still catch them, had already been beaten 1-0 by Iraklis, so handing Olympiakos the title.
It is the 34th championship in Olympiakos' history and qualifies them for next season's Champions League.
It also means they are one game away from collecting their 12th league and cup double - they face AEK Athens in the Greek Cup final on May 6.
Olympiakos began the game as if they had already started the celebrations and were soon trailing to a seventh-minute penalty from Angelos Digkozis.
Predrag Djordjevic levelled the scores in the 54th minute but four minutes later Stathis Aloneftis latched on to a mistake by Anastasios Pantos to put Larissa back in front.
Panathinaikos went into their game in Thessaloniki knowing that anything less than a win would present the title to their biggest rivals but they were immediately under pressure from fourth-placed Iraklis.
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In the 33rd minute, however, Iraklis deservedly took the lead when Epalle sent over a pinpoint cross from the right which Argentine Jose Esteban Herrera headed home.
Defeat for Panathinaikos also means they have slipped behind AEK in the race for the second Champions League spot.
Two goals in the first half from their international pair Costas Katsouranis and Nikos Lymberopoulos gave AEK a 2-0 win over OFI Crete who are now one point behind Panionios in 14th place and in serious danger of relegation.
The two bottom clubs Akratitos and Kallithea had their demotion confirmed even though Kallithea produced a battling 2-1 win over Xanthi.