Diarra won the Carling Cup for Chelsea against Arsenal
Arsenal completed a late move to sign midfielder Lassana Diarra from Chelsea for an undisclosed fee.
The Gunners say the 22-year-old Frenchman, who can play at right-back, has signed a long-term deal.
The former Le Harve player said: "I have great respect for the manager Arsene Wenger and am attracted by the style of football that the team plays.
"Of course I am excited by what the future holds and keen to play my part in helping Arsenal fight for trophies."
Diarra, who has played for France in their Euro 2008 qualifying campaign, will wear the number eight shirt.
Wenger added: "Lassana is a multi-functional player, making him a great addition to our squad. Not only is he hard working, he has a creative edge and is comfortable playing in the middle of the pitch or at right-back."
Squad Number: 19
Place of Birth: Paris, France
Date Of Birth: 10/ 03/ 1985
Weight: 10st 10lbs
Yellow Cards: 10
Red Cards: 0
Minutes on Pitch: 1419