The Brazilian’s contract with Chelsea expired after their Champions League defeat by Bayern Munich last Saturday.
Willian, who turned 32 on Sunday, has signed a three-year deal that will see him earn around £100,000 a-week, although that could rise with appearance and bonus clauses.
In seven seasons at Chelsea, he won the Premier League twice, the FA Cup, the Carabao Cup and the Europa League, but he is convinced he can add to that collection with the Gunners.
“I decided to join because Arsenal are one of the biggest clubs in the world. They have a new project with Mikel Arteta,” said Willian. “I think this club deserves to shine again and I want to be a part of this project to be a part of the Arsenal family is amazing. I’m very happy to do that.
“[I like] the way they play, everything is amazing, the stadium, and I think that with Arteta, Arsenal have a great opportunity to fight again for titles in the Premier League and in Europe, so I’m very happy because of that.
“When I talked to Mikel, the conversation was good; he gave me confidence to come to Arsenal. He said a lot of good things to me and that’s why I have come to Arsenal.”