Episodes in the fourth season of the show have an average gross audience of 18.4 million, including repeat showings, video on demand and mobile streaming, which broke the record of 18.2 million set by the 2002 season of the mobster drama, "The Sopranos."
Video on demand and delayed viewing habits have changed significantly since 2002, with more options being open to television audiences.
Season three of the show averaged 14.4 million viewers per episode. Two episodes of the fourth season have yet to be aired, with the finale set for June 15. HBO has already committed to making seasons five and six.
The series on Time Warner Inc-owned HBO network features dragons, castles and dragons. It is based on the best-selling books by fantasy and science fiction writer George R.R. Martin about fictional, medieval families fighting for control of a mythical land.
The show, which first aired in 2011, stars Peter Dinklage, Lena Headey, Emilia Clarke and Maisie Williams, and is sold to more than 200 markets outside the United States.