The Barcelona forward earned USD 127million (£99.8million) over the last 12 months, USD 92million in salary and winnings and USD 35million in endorsements, according to the American business magazine.
Juventus striker Ronaldo earned USA 109million (£85.6million), with a third footballer, Brazil's Neymar, third with USD 105million (£82.5million).
Mexican boxer Canelo Alvarez is fourth on the list with USD 94million (£73.8million), with Roger Federer fifth with USD 93.4million (£73.3million).
Serena Williams was the only female athlete in the top 100, in 63rd position with USD 29.2million (£22.9million).
Lewis Hamilton and Anthony Joshua were the highest-placed Britons, sitting at joint 13th on USD 55million (£43.2million).
Manchester United midfielder Paul Pogba was the highest-earning Premier League star, his USD 33million (£25.9million) putting him at 44th place.
There was no place in the top 100 for last year's highest earner, Floyd Mayweather.
The top 10 highest-paid athletes in the world in 2019, according to Forbes:
1 Lionel Messi USD 127million (£99.8million)
2 Cristiano Ronaldo USD 109million (£85.6million)
3 Neymar USD 105million (£82.5million)
4 Canelo Alvarez USD 94million (£73.8million)
5 Roger Federer USD 93.4million (£73.3million)
6 Russell Wilson USD 89.5million (£70.3million)
7 Aaron Rodgers USD 89.3million (£70.1million)
8 LeBron James USD 89million (£69.9million)
9 Stephen Curry USD 79.8million (£62.7million)
10 Kevin Durant USD 65.4million (£51.3million)
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