Right inner bicep - sanskrit which reads "Anuugacchati Pravaha" - "Go With The Flow"
A lot of people think that Russell has a great number of tattoos due to his various movie roles, particularly Forgetting Sarah Marshall. All of these tattoos were fakes and were applied for the role. He and his wife, Katy Perry have matching sanskrit tattoos in the same place.
Source: Rolling Stone Magazine US (June 2010 issue), The Mirror