The Swiss midfielder ended his seven-year spell with the North London giants and joined Bayer Leverkusen in a deal worth £21million.
He has spent seven seasons with the Gunners in which he made 297 appearances for the club bagging 23 goals and 29 assists.
He will now return to the Bundesliga as he spent four seasons with Borussia Monchengladbach before joining Arsenal in 2016.
Xhaka struggled at Arsenal during his time but his fortune has improved since Mikel Arteta took over the managerial role.
His departure comes due to the imminent arrival of Declan Rice who is expected to arrive in a few days.
Xhaka said: “I know the league inside out and I watched it when I was in London. Bayer 04 is a club with an impressive history and ambitious targets.
“Above all, I see it as a club with a great future. Discussions with the management have been incredibly motivating. Everybody here is ambitious and wants to achieve something – I’m very much looking forward to the next few years.”
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