Former welterweight king Mayweather, who came out of retirement to face McGregor in Las Vegas, topped the rankings for the 4th time in 7 years.
The list is dominated by athletes from America's NFL, NBA and Major League Baseball competitions, although the top five athletes - Mayweather, Messi, Cristiano Ronaldo, Conor McGregor and Neymar - are from either football or combat sports. Mayweather easily won that bout, but McGregor has the solace of ranking number four on the Forbes list, with estimated earnings of $99 million.
"Boxing's biggest night of 2017 helped Mayweather and McGregor earn almost $400 million combined", Kurt Badenhausen, senior editor, Forbes Media said in a statement on Tuesday.
Serena Williams would likely have made the cut, but she had been off the court during her pregnancy and maternity leave. Endorsement incomes include estimations of an athlete's sponsorship deals, appearance fees, and licensing income.
Completing the top 10 were basketball's LeBron James ($85.5M), tennis' Roger Federer ($77.2M), basketball's Stephen Curry ($76.9M) and quarterbacks Matt Ryan ($67.3M) and Matthew Stafford ($59.5M).
Forbes's sporting rich list is created after calculating an athlete's earnings from prize money, salaries and bonuses in the year to 1 June 2018.
U.S. boxer Floyd Mayweather came top of the list having reportedly earned $275m (£205m) for his boxing match against UFC star Conor McGregor last August alone.
Irish mixed martial artist McGregor ($US99 million; $130 million) and Paris St Germain's Neymar ($US90 million; $118 million) were fourth and fifth, respectively, but for the first time since 2010 there was not a single woman on the list.
No Australian athletes made the top 100 this year. Close on the heels of Messi is another football legend, Cristiano Ronaldo.
Between them, the top 100 earned a collective $3.8bn during the past 12 months, which represents an increase of 23% from 2017.
To check out the full list, you can head to the link here.