The agency picked up the business following a pitch against undisclosed agencies. The review was handled by Creative Brief.
Play is the lager brand's first retained digital agency and will work across its entire online account, including advertising, branded entertainment and web build.
Until now, digital duties for Foster's have been handled by the company's direct marketing shop, EHS Brann Cirencester.
The company won the DM account from Harrison Troughton Wunderman in August last year, but did not repitch for the digital part of the business.
Play will now be briefed with targeting young adult drinkers who have migrated away from TV by building a dominant digital presence.
The agency will also work closely with M&C Saatchi, its sister agency that handles above-the-line duties for the Foster's brand.