Willian who left Chelsea as a free agent, will wear the number 12 shirt at the Emirates.
His deal with the Gunners is believed to be worth around £100,000-a-week.
Willian has made more than 600 club appearances in a career spanning 13 seasons.
He began his career at Brazilian side Corinthians, before joining Shakhtar Donetsk in August 2007.
Four years later, he briefly signed for Anzhi Makhachkala before joining Chelsea, where he won the Premier League and League Cup in his second season.
During his seven years at Stamford Bridge, Willian won two Premier League titles, an FA Cup and the Europa League. He made 339 appearances, with 63 goals and 56 assists in that time.5
