Floyed Mayweather, aka 'Money', earned $275 million from last year's fight against Irish Mixed Martial Arts champion McGregor, whose cut of the purse elevated him to fourth spot on the U.S. business magazine's list.
The fight pushed Mayweather's career earnings to $1 billion, joining American basketball great Michael Jordan and golfer Tiger Woods as the only athletes to have reached 10-figures in earnings, Forbes said.
Portuguese soccer player Cristiano Ronaldo, who topped the rankings for the last two years, slipped to third in the standings behind his Argentine rival Lionel Messi.
Brazil's Neymar, who became the world's most expensive soccer player after his $260 million move from Barcelona to Paris St Germain last August, was listed fifth.
Swiss tennis great Roger Federer, basketball players LeBron James and Stephen Curry and National Football League quarterbacks Matt Ryan and Matthew Stafford rounded out the top 10 of the 100 wealthy athletes.
The were no women in the list, with tennis icon Serena Williams, the sole female representative last year, having dropped out of the rankings after taking a break from the sport due to the birth of her daughter.
Top 20 (lists under athlete/nationality/sport/estimated earnings):
1. Floyd Mayweather (U.S.) Boxing $285 million
2. Lionel Messi (Argentina) Soccer $111 million
3. Cristiano Ronaldo (Portugal) Soccer $108 million
4. Conor McGregor (Ireland) MMA $99 million
5. Neymar (Brazil) Soccer $90 million
6. LeBron James (U.S.) Basketball $85.5 million 7. Roger Federer (Switzerland) Tennis $77.2 million 8. Stephen Curry (U.S.) Basketball $76.9 million 9. Matt Ryan (U.S.) American football $67.3 million 10. Matthew Stafford (U.S.) American football $59.5 million 11. Kevin Durant (U.S.) Basketball $57.3 million 12. Lewis Hamilton (Britain) Formula One $51 million 13. Russell Westbrook (U.S.) Basketball $47.6 million 14. James Harden (U.S.) Basketball $46.4 million 15. Canelo Alvarez (Mexico) Boxing $44.5 million 16. Tiger Woods (U.S.) Golf $43.3 million 17. Drew Brees (U.S.) American football $42.9 million 18. Sebastian Vettel (Germany) Formula One 42.3 million 19. Derek Carr (U.S.) American football $42.1 million 20. Rafael Nadal (Spain) Tennis $41.4 million