According to news site TorrentFreak.com's new survey, one episode of the programme, which stars Peter Dinklage and Emilia Clarke, was illegally downloaded by fans around the world a whopping 4.28 million times.
Among the other top 10 targetted shows were Claire Danes' Homeland, Breaking Bad, Dexter, and The Walking Dead.
Editor Ernesto Van Der Sar tells BBC.com most viewers resort to peer-to-peer sharing because a handful of the shows aren't available to them.
He says, "Not all of the people who pirate do it because it's free - availability is also a big factor. Most of the titles in the top 10 list are behind paywalls and are not distributed very widely."
Meanwhile, Project X beat Mission: Impossible - Ghost Protocol to the top of TorrentFreak's most pirated movies list.