Mostefa makes the move after playing two games during Algeria's run to the last 16 of the World Cup in Brazil.
The defensive midfielder, who can also operate as a centre-back, spent three years with Corsican side Ajaccio after joining them from Nimes in 2011.
Mostefa could not prevent Ajaccio from suffering the ignominy of relegation from Ligue 1 last season, as they finished bottom with just 23 points to their name.
But Mostefa will be playing in the French top flight in the coming season after becoming the latest recruit of new Lorient coach Sylvain Ripoll, who took over from Christian Gourcuff in May.
Ripoll will preside over a much changed side in his maiden season in charge.
Ghana forward Jordan Ayew and former France Under-21 international Benjamin Jeannot are among the other new faces at Stade du Moustoir.
First-team regulars Jeremie Aliadiere, Kevin Monnet-Paquet and Gregory Bourillon have left the club, but the 30-year-old Mostefa is excited by the prospect of playing for the Brittany club.
"I was interested in the project of the club," Mostefa told Lorient's official website.
"To leave Ajaccio for Lorient is a great step for me in my career. I am very happy to be here.
"I am ambitious and really want to make a great season with Lorient."
Mostefa could make his Lorient debut at Monaco on Sunday.
