The 29-year-old was released by Arsenal at the end of last season after nine years at the London club in which his progress was severely hampered by injuries.
He was offered the chance to stay in the Premier League with West Brom but turned down a move to The Hawthorns.
Instead he will be heading back to France after playing only two games in the last two seasons for Arsenal.
“Olympique de Marseille and Abou Diaby have this Tuesday evening reached an agreement in principle for the international midfielder to join the club,” Marseille said on their website.
The 29-year-old joined Arsenal from French club Auxerre in 2006 but his last match for the Gunners was September 2014.
Arsenal manager Arsene Wenger admitted he has “enormous respect” for Diaby but added: “Each time he came back, we had to start again from zero because of another injury.
“He is not a player who is fragile but just a victim of a violent tackle which went unpunished.”