Ayew, whose brother Andre enjoyed a loan spell with Lorient in the 2008-09 season, came through the youth system at Marseille but fell out of favour last term.
He spent the second half of the campaign on loan at Sochaux, where his five goals were not enough to prevent the club suffering relegation.
While Sochaux will begin the new season in Ligue 2, Ayew has ensured he will continue to play in France's top flight.
"The style and the quality of play offered here suits me perfectly," Ayew told Lorient's official website.
"I am also very impressed by the infrastructure.
"I will try to bring my experience to this group in order to achieve the collective goals of the club."
Arnaud Tanguy, Lorient's director general, stated that the arrival of Ayew represented "a recruitment that reflects the desire of the club to continue its progress."
Lorient finished eighth in Ligue 1 last season, two places behind Marseille.
Ayew, 22, made 111 league appearances for Marseille, scoring 14 goals.
