Nacho Monreal has completed his move from Arsenal to Real Sociedad to end a six-and-a-half year spell in north London.
The deal is worth a reported 250,000 euros (£225,000), with the 33-year-old left-back signing a two-year deal in San Sebastian.
Monreal joined Arsenal from Malaga in January 2013 and won three FA Cups and three Community Shields as a Gunner, scoring 10 goals in 250 appearances.
"We would like to thank Nacho for his contribution to the club and everyone at Arsenal wishes him the best for his future," Arsenal said in a statement.
Monreal's move comes on the eve of the first north London derby of the season, and leaves Arsenal with a defensive shortage in the short-term.
Though they signed a long-term replacement in the form of Kieran Tierney in the summer, the Scotland international is out injured and not expected back until the end of September. Rob Holding and Hector Bellerin are also sidelined.
Monreal had started all three Premier League games so far this season, captaining the side against Burnley, but Sead Kolasinic is now the most likely candidate to slot in on Sunday.
The Bosnian has made only one substitute appearance so far following security concerns after he and team-mate Mesut Ozil were the subject of an attempted armed robbery.