"We will make an announcement tonight. Yes, it will be a positive announcement. Yes, it will go through," Sky Sports News quoted Arsenal manager Arsene Wegner as saying.
The news follows a day of intrigue and confusion after conflicting reports from Zenit first said the deal was off and then back on again.
The Zenit forward had been in London all day and was earlier reported to have passed an Arsenal medical.
It is believed that the deal was closed for 12 million pounds ($17 million), and will keep Arshavin in London until 2012.
The transfer window closed at 17:00 GMT. However, an extension was made for deals that had been agreed on "in principle" due to the snowstorms that paralyzed much of Britain on Monday.
Arshavin, 27, emerged from the shadows of Russian soccer last summer after unexpectedly leading the national side to the semi-finals of Euro 2008.
The Russian could make his debut for Arsenal this weekend against Tottenham Hotspur, the side that Roman Pavlyuchenko, Russia's only other representative in the English Premier League, has scored 12 goals for in 26 games since signing for Spartak Moscow in the summer.
Media reports say that Arshavin will wear the No. 13 shirt for Arsenal.
