The HBO show was the most illegally-watched programme of the year, with one episode being downloaded 4.28 million times via various BitTorrent clients, figures from TorrentFreak show.
The figure outstripped the official estimated US TV ratings of 4.2m. In second place on the list was an episode of Dexter, whose 3.85m downloads dwarfed the estimated US viewing figure of 2.75m.
"The majority of TV-show piracy, more than 80%, occurs outside the US and lacking availability is the top reason why people turn to BitTorrent," the site argued.
"For chart leader Game of Thrones this is no different. Airing delays and HBO's choice not to make it widely available online are two of the top reasons why so many people pirated the show."
It noted the show's popularity in Australia, where there is a week-long delay in the broadcast of new episodes, and the lack of availability of a standalone HBO subscription offer in the US.
TorrentFreak claimed that TV companies have the challenge of coming up with a business model that lets viewers "consume what they want at any given time, without losing existing revenue in the process".
"When that puzzle is solved worldwide the number of unauthorized downloads will be pretty much insignificant," it added.
The most-downloaded TV shows of the year via BitTorrent were as follows, with download figures shown for a single episode and estimated total US viewers in brackets
1. Game of Thrones - 4,280,000 (4,200,000)
2. Dexter - 3,850,000 (2,750,000)
3. The Big Bang Theory - 3,200,000 (15,820,000)
4. How I Met Your Mother - 2,960,000 (10,140,000)
5. Breaking Bad - 2,580,000 (2,980,000)
6. The Walking Dead - 2,550,000 (10,870,000)
7. Homeland - 2,400,000 (2,360,000)
8. House - 2,340,000 (9,780,000)
9. Fringe - 2,280,000 (3,120,000)
10. Revolution - 2,130,000 (11,650,000)
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