The pay channel said the 17.4 million viewers who watched Sunday's episode either on TV or online represent a season-opening high for the fantasy saga.
HBO Now also posted its biggest streaming night ever, the channel said Monday.
The episode topped the 16.1 million who saw the seventh-season premiere and the 16.9 million who watched that season's finale.
Reflecting increasing audience fondness for streaming, HBO saw about a 50 percent increase in online viewing compared to last season's finale. In comparison to the season-seven premiere, the streaming audience nearly doubled.
"Game of Thrones" ultimately averaged 32.8 million viewers per episode last season in cumulative TV and online viewership, HBO said.