The fantasy series, which goes out on Sunday nights on HBO in the US, has overtaken The Sopranos, which set the previous audience record for HBO in 2002.
Game of Thrones garners 6-7 million live viewers, but the overall viewing number is considerably higher when viewers watching it later in the week and on HBO Go are included.
HBO claims an average gross audience of 18.4 million for the current fourth season of Game of Thrones, which draws to a close next Sunday a sharp increase on the 14.4 million average commanded by season three. The Sopranos averaged 18.2 million.
HBO has already greenlit fifth and sixth seasons of the series, which is based on the George R R Martin book series.