Ronaldo, 35, is ranked at No. 4 in the business magazine's list of the world's highest-paid figures in 2020 -- behind only Kylie Jenner, Kanye West and Roger Federer -- with analysis suggesting he brought in $105 million before tax over the past year.
According to Forbes, he is just the third athlete to do so while still active, following Tiger Woods and Floyd Mayweather, and the first to do so playing a team sport.
ESPN has approached Ronaldo's representatives for comment.
The list judges Messi to have made $104 million this year. The figures mark the latest, off-the-pitch chapter in a rivalry that has defined football over the past decade.
The pair won 10 consecutive Ballon d'Or trophies between them from 2008 to 2018. In that time, Messi's Barcelona came out on top domestically with seven league titles, while Ronaldo's Real Madrid won the Champions League on four occasions.
Ronaldo has expanded his fame beyond football with his CR7 lifestyle brand and a hotel chain, as well as a museum in his native Madeira.