On social media, Koscielny explained, "I have spent wonderful years with this club, nine years during which I learned, evolved, grew up.
"I see this departure like a new turning point in my life as a player but also in my life as a man and a family man.
"However I hope you can understand, and in any case I cannot be thankful enough for these years past at Arsenal.
"I am aware of everything that the club and you supporters have brought to me. But it is time for me to go home so today my will is to join the @girondins (Bordeaux)."