The Scotland match started at 2215 local time in Japan and 19.5 million people, around 15 percent of the population, stayed up to watch it, the television rights-holding broadcaster said.
Less than one percent of Japanese people viewed the opening match of the tournament between England and Fiji.
"These broadcast numbers from Japan are truly incredible and show the difference that can be made with a great performance by the national side," World Rugby chief executive Brett Gosper said in a statement.
"The viewership figures in Japan have been improving anyway but the victory over the Springboks has given the game a welcome shot in the arm that comes at a perfect time as Japan prepares to host Rugby World Cup 2019."
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Japan, who lost 45-10 to the Scots, next face Samoa on Oct. 3 in their third Pool B match.
(Reporting by Ed Osmond)
