The Chile international underwent a medical in London on Thursday before being officially announced as an Arsenal player, with manager Arsene Wenger thrilled to have recruited one of the stars of World Cup 2014.
And Walcott, who is likely to compete with Alexis for a position in the Gunners' attacking midfield area, took to Twitter to share his excitement.
What a Signing!!!!! Can not wait and looking forward to playing with Alexis Sanchez!!!!! #COYG— Theo Walcott (@theowalcott) July 10, 2014
Alexis becomes the FA Cup winners' first signing of the summer and, though Mathieu Debuchy's arrival from Newcastle is imminent, Wenger is also weighing up a move for Brazil winger Bernard.