The 29-year-old spent nine years at Arsenal after joining from Auxerre in 2006, but made just two appearances over the past two seasons of an injury-plagued career.
West Brom head coach Tony Pulis confirmed last week the club had offered Diaby a chance to stay in the Premier League.
But Diaby, a free agent after being released by Arsenal earlier this summer, turned down a move to the Hawthorns and has returned to France.
Diaby was a mainstay in Arsenal’s midfield between 2007 and 2010, but a string of injuries restricted him to only 22 first team appearances since.
He has 16 caps for France, scoring one goal.