Nacho Monreal has left Arsenal after more than six seasons and signed a two-year deal with Real Sociedad.
The 33-year-old started each of the Premier League side's opening three fixtures but Unai Emery admitted his departure was possible following last weekend's 3-1 loss to Liverpool.
An experienced and versatile defender, Monreal moved to north London from Malaga in January 2013 and made 250 appearances in all competitions, scoring 10 goals.
Neither club has confirmed the transfer fee involved.
"We would like to thank Nacho for his contribution to the club and everyone at Arsenal wishes him the best for his future," read a statement from the Gunners.
"The transfer is subject to the completion of regulatory processes."
Monreal's days at Arsenal appeared numbered when the younger Kieran Tierney, a left-sided defender of similar profile, arrived from Celtic on deadline day.
Head coach Emery further strengthened his options at the back by signing David Luiz and William Saliba, who is spending the season on loan at Saint-Etienne.
The Spain international is due to be presented by Sociedad on Monday and could debut in front of his new fans when Imanol Alguacil's men host Atletico Madrid after the international break.