FarmTown creator SlashKey is warning players that the site has been hit with malicious software.
The virus resides in advertisements displaying fake security warnings with the aim of persuading users to provide their credit card details to buy anti-virus software.
"We believe at this time that it is harmless to your computer and a result of one or more of the ads on the site, but you should NOT follow any links to any software claiming to 'clean your system'," says the company.
"If you suddenly get a warning that your computer is infected with viruses and you must run this scan now, do not click on the link, close the window immediately."
"Such malicious advertising (or malvertising as it is known) has been the vector for other infections in the past, including attacks against the readers of the New York Times and Gizmodo," says Graham Clueley of security firm Sophos..
"What makes this attack all the more serious, of course, is the sheer number of people that regularly play Farm Town, and that - in all likelihood - they might not be as tech-savvy as the typical Gizmodo reader, and thus more vulnerable to falling for the hackers' scam."
SlashKey is asking users to report any instances of the virus, but Clueley thinks it's not going far enough.
"Itt might be sensible for the company to disable third-party adverts appearing alongside Farm Town until the problem is fixed," he says.
"It may not be Farm Town's fault that a third-party advertising network is serving up malicious ads, but doing anything less is surely showing a careless disregard for the safety of its players."