Once repeats were taken into account, the finale for the fourth season was seen by 9.3 million people.
Total viewing figures, including video on demand and mobile streaming have not yet been released.
Game of Thrones is now HBO’s most popular series, surpassing The Sopranos.
The average audience during series four, including catch-up services, has been 18.6 million per episode. The Sopranos’ audience peaked at 18.2 million in 2002, giving it the HBO record until now.
The show is engaged in a ratings war with zombie thriller The Walking Dead, which attracted 15.7 million viewers for its season finale in April.
However, broadcaster AMC said its audience jumped to 28 million once repeats and on-demand figures were taken into account.
The final Game of Thrones episode, The Children, saw many of the show’s leading characters making key decisions which will impact them in future episodes, while several met a typically grisly end.
Season five has already been confirmed.
Game of Thrones TV series is based on the books, A Song of Ice and Fire by George RR Martin, set in the mythical, medieval land of Westeros.
In the north, a massive wall of ice keeps out barbarian “wildlings” and zombie-like “white walkers”. South of the wall, there is a bitter war between rival claimants for the Iron Throne and rule of the seven kingdoms of Westeros.
Meanwhile, a princess exiled to distant lands with three dragons has rallied an army of freed slaves.
Game of Thrones is full of intense political intrigue, with war and its aftermath depicted in brutal detail. The show first aired in 2011 featuring actors including Peter Dinklage, Charles Dance, Lena Headey, Emilia Clarke and Maisie Williams.
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