The figure, which captures live and as well as all time-shifted viewing a week after launch, represents a 1.2 million increase on the viewing number for the season four opener.
It is down, however, on the season four gross average of 19.1 million. HBO released the first live-plus-7 ratings for season five yesterday.
The hit fantasy series is based on the George R. R. Martin book series. The TV show, one of the most illegally downloaded in the world, is going out day and date internationally on several pay TV platforms.
On Sky Atlantic in the UK, the season five opener won 1.57 million live viewers, a 6.7% audience share and beat free-to-air channels BBC Two and Channel 5. It was the most popular show in multichannel homes, according to Sky.