Willian has been handed the No.12 shirt after completing the switch from Chelseato Arsenal on a free transfer.
The 32-year-old is believed to have signed a three-year deal worth £220,000-a-week at the Emirates after failing to come to an agreement with the Blues.
Willian brings plenty of experience to Mikel Arteta's side having won two Premier League titles, an FA Cup and the Europa League over his seven-year spell at Stamford Bridge.
Upon confirming the deal, Arsenal also announced that the Brazilian will wear the No.12 shirt, mostly recently donned by Stephan Lichtsteiner and Olivier Giroud, who is now a Chelsea player.
Willian had offers from several clubs, but chose to be part of Mikel Arteta's exciting project at Arsenal.
Technical director Edu told the club's official website that his compatriot will bring a number of good qualities to his new post.