The Russian forward joined "on a long-term contract for an undisclosed fee," the club said on its website.
Prior to the announcement, it was not clear if the transfer would be sanctioned by the FA due to the deal being completed after the closure of Monday's transfer window.
The English FA's director of communications, Adrian Bevington, said earlier on Tuesday that it could take up to three days for the sport's governing body to rule on the transfer. He told Russia's sports.ru website that the odds of the 27-year-old Zenit St. Petersburg forward becoming an Arsenal player were "50-50."
The transfer window closed at 17:00 GMT on Monday without any definite news on protracted talks between Arsenal and Zenit. The Russian club's press officer then told Radio Zenit that the deal was off, only for Zenit general director Maxim Mitrofanov to insist that talks were continuing. Media reports then said that a deal had been clinched to make Arshavin an Arsenal player until June 2012.
However, in line with a Premier League ruling on the transfer deadline extension, the clubs were allowed to prove that the snowstorms that paralyzed much of Britain on Monday prevented the deal going through on time.
The Russian playmaker could make his debut for Arsenal this weekend against his only compatriot in the English Premier League - Tottenham Hotspur striker Roman Pavlyuchenko, who has scored 12 goals for in 26 games since signing from Spartak Moscow in the summer.
Arshavin scored 51 goals in 232 appearances Zenit, and has found the net 15 times in 41 games for his country, which he inspired to the semifinals of Euro 2008, being named to UEFA's team of the tournament.
