The 32-year-old was a free agent after leaving Chelsea, having spent seven years at Stamford Bridge.
He had rejected the offer of a two-year extension with the Blues, and has now joined Chelsea’s London rivals.
The Brazilian is delighted to have made the switch across London describing the Gunners as “one of the biggest clubs in the world”.
“I’m very excited to become an Arsenal player – I can’t wait to start training and playing with my team-mates,” he said.
“I decided [to join] because Arsenal is one of the biggest clubs in the world. They have a new project with Mikel Arteta.
“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 come to try to win trophies, because this club deserves to win again, and I think you have to do the best as possible to win every game.
“I always train hard and go on the pitch giving 100 per cent to help this team to win games and to win titles as well.”
Boss Mikel Arteta admitted he was impressed by Willian’s desire to join.
“I believe he’s a player that can really make a difference for us,” he said.
“We have been monitoring him for the past few months, we had a clear intention to strengthen in the attacking midfielder and the winger positions. He is a player that gives us a lot of versatility, he can play in three or four different positions.
“He has the experience of everything in the football world but still has the ambition to come here and contribute to bring the club where it belongs.”I have been really impressed with all the talks I have had with him and how much he wanted to come.”
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