Mayweather earned $275m for his boxing match last August against UFC star Conor McGregor, who ranked number four on the US business magazine’s list.
Cristiano Ronaldo – top for the past two years – drops to third, with Lionel Messi edging him out of second place.
Curiously there are no women on the list, after the previous sole entrant, Serena Williams, dropped out of the chart.
Mayweather – one of whose nicknames is Money – saw his sports earnings added to by $10m from endorsements.
Boxing’s biggest night of 2017 helped Mayweather and McGregor earn nearly $400m combined.
It is basketball players however that dominate the top 100 overall thanks to a soaring salary cap triggered by the NBA’s $24bn TV contract.
NBA stars lead with a record-topping 40 basketball players among the top 100. American Football was the next most-represented sport with 18 players, followed by baseball with 14 and football with nine.
Forbes’ lists of top-earning athletes have long been testosterone heavy. But there has always been at least one female athlete, and as many as three, who qualified since the list was expanded to at least 50 names in 2010. This year, not a single female athlete made the list.
Floyd Mayweather, Cristiano Ronaldo and Tiger Woods – at number 16 this year – are the only three athletes to top the best-paid list over the past 18 years.
Players from 11 different sports are featured in the list, and from 22 different countries.
Americans dominate the placings with 66 athletes, while the UK has five, led by F1 driver Lewis Hamilton at number 12 with $51m.