The French midfielder has signed for the Stade Velodrome outfit, with the first images of him with the team’s strip emerging.
The 29-year-old was being widely tipped as a player capable of replacing Gunners legend Patrick Vieira in the team’s midfield and over the years showed signs of considerable ability.
However, Diaby suffered from repeated injuries that have seen him on the treatment table for prolonged periods over the last number of years - making him something of a forgotten man at the Emirates Stadium.
The 16-times capped France international will hope to get his career back on track with Marcelo Bielsa’s men, but in reality the most important thing will be for him to get fit and be able to play.
Diaby was out of contract earlier this summer, with Arsene Wenger and the powers that be at the North London club not extending his deal due to an inability to prove his fitness.
The ex-Auxerre man leaves the Emirates Stadium after nine years with Arsenal, but in that time he featured in only 125 Premier League games.