Bordeaux announced the signing of Koscielny on the club homepage. Koscielny, who has played for Arsenal since 2010-11, has returned to the French stage after nine years to Bordeaux.
Koscielny played 255 Premier League appearances for Arsenal and scored 22 goals.
Koscielny, who played 51 match for France, has won three FA Cup championships at Arsenal. Koscielny was controversial, throwing off his former Arsenal uniform in a video released after joining Bordeaux.
Bordeaux, meanwhile, will play the opening game of the French League 1 in the 2019-20 season against Angers on the 10th.