Japan's Rugby World Cup win over Samoa was watched by 25 million people, which represents nearly 20% of the nation's population.
The landmark viewing figure, revealed by the sport's governing body
, eclipsed the previous record of 20.7m viewers for the 2007 World Cup semifinal between France and England on French broadcaster TF1.
"This is a significant result for the game in Japan," World Rugby chief executive Brett Gosper said in a statement. "It shows that the general population there appreciate the sport and the amazing performances of their national team over the past few weeks.
"Japan have set this entire tournament alight with how they have performed in all their games."
The tenacious "Brave Blossoms" caused one of the biggest upsets in global rugby history when they defeated two-time champions South Africa
in their opening game of the 2015 tournament.
Japan must win their final Pool B match against USA on Sunday in Gloucester, and hope other results go their way, to reach the quarterfinals of a World Cup for the first time.
The rugby fever sweeping through Japan has led some to rename the country the "land of the rising scrum."
The playful moniker is fitting as Japan will host the 2019 World Cup, the first time the prestigious global event has been held in Asia. In May, Japan's women were also crowned Asian champions for the first time.
"If you went (to Japan) today you would see rugby everywhere, which is not the normal case," Japan's Australian coach Eddie Jones told CNN.
"There's usually baseball, J-League soccer, then sumo ... rugby comes in eighth or ninth in terms of popularity. But at the moment we're batting two or three which is quite an elevation."
World Rugby chose Japan as the hosts of the 2019 World Cup to help grow the game in Asia and the new TV audience figure shows the sport's chiefs are on the ball.
"This boost in interest could not be better timed as the Rugby World Cup will venture outside of traditional rugby heartlands for the first time in 2019," added Gosper.
"It really has given the event a major shot in the arm."