The Italy goalkeeper has signed on a season-long loan.
"He comes to the Club at the top of his game. It was a very good deal for us at a good moment, I think it was a no-brainer as well," he told Arsenal Player.
"With two goalkeepers available for the Premier League and all the competitions we face, we felt we were a bit short because sometimes you have injuries and sometimes you need to rest one of the goalkeepers.
"Viviano is a highly-rated goalkeeper in Italy so we have three goalkeepers that are very compact quality-wise."
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