The announcement comes in advance of "aggressive growth plans" for the company, which was founded in 1982 by the billionaire owner of Newcastle United football club, Mike Ashley.
Arena won the business following a head-to-head against MEC Manchester, which has worked with the client on previous projects.
Sportsdirect.com supplies a range of equipment, clothing, footwear and accessories from brands including Reebok, Nike, Adidas, Slazenger and Umbro.
The bulk of the company's advertising spend is currently directed towards TV activity, which is created by Mood Media.
Vincent Smith, the Sportsdirect.com head of marketing, said: "We were impressed with Arena's ideas and the agency's understanding of our business challenges."