According to Forbes magazine, Osaka, 22, a two-time Grand Slam champion, made £30.7m in prize money and endorsements over the past 12 months. The figure is also the highest ever for a female athlete, passing the $29.7 million payday Maria Sharapova earned in 2015, according to the report.
"Combine that with being youthful and bicultural - two attributes that help her resonate with younger, global audiences - and the result is the emergence of a global sports marketing icon".
That was £1.15m more than the amount earned by 38-year-old Williams.
Osaka is No. 29 overall, with Williams at No. 33, on Forbes' annual list of the 100 top-earning athletes.
Osaka burst on the scene in 2018 when she beat Williams at the U.S. Open for her first major title at 20 years old.
The complete Forbes list, due to be released next week, has not featured two women since 2016, the magazine says.
Japan's Naomi Osaka, seen here during a women's singles match on day five of the Australian Open tennis tournament in Melbourne on January 24, 2020, has become the world's highest-paid female athlete.
That made her the first Asian player to reach number one in the rankings and earned her more than $4 million in prize money, although her plans to represent her country and brands including United States multinational Procter and Gamble at this summer's Olympic Games in Tokyo have been put on hold by the COVID-19 pandemic.
Female tennis players have always occupied top spot on Forbes's women's list since 1990 with either Williams or Russian Sharapova leading the way from 2004.
Osaka was a popular endorsement figure in Japan ahead of the now-postponed Tokyo Olympics and figures to remain a sponsorship dream through next year's rescheduled Games.