The 27-year-old midfielder has joined the Greek champions from French side Nice for an undisclosed fee, signing a one-year contract with the option to extend for a further season, the club said.
Olympiacos were able to make the signing even though the European transfer window officially closed on August 31 as the deadline for Greek clubs is September 15.
"I am very happy to be in Greece and join Olympiacos which is certainly a very big club," Diakite told the club website.
"The opportunity to play in the Champions League certainly played an important role since it is the dream for every footballer to play in that competition and so I did not hesitate to make the decision."
Diakite made more than 150 appearances for Nice after joining the club in 2006.
The Mali international was immediately added to the Olympiacos squad list sent to UEFA on Tuesday. The club face Arsenal, Montpellier and Schalke in Group B of the Champions League.