The 22-year-old Ghana international has signed a four-year contract with the Ligue 1 club after struggling for first-team opportunities at the Stade Velodrome.
Ayew told the club's website: "The style and quality of play here attracted me.
"I was also impressed by the infrastructure of FCL. It will allow us to work well every day and progress.
"I will try to bring my experience to the squad to achieve the goals set for the club."
Lorient's general director Arnaud Tanguy added: "We are very happy with this arrival. It is a signing which shows the club's ambition to keep progressing."
Ayew's elder brother, and erstwhile OM team-mate, Andre spent the 2008-09 season on loan at Lorient while Jordan made his professional debut against them the following season.