He earned $80m (£51m) thanks to blockbusters like Avengers: Age of Ultron, according to the list.
For the first time, the list also includes actors who work outside the US movie industry, meaning the top 10 includes three Bollywood stars.
Daniel Craig was the only British actor to feature, at number 15 jointly with Australian actor Chris Hemsworth.
They each earned $27m (£17m) according to Forbes.
In compiling the list, the number crunchers measure earnings before subtracting management fees and taxes over the past year.
Forbes described it as "the first truly global earnings tally of leading men", adding that the 34 stars on the full list "earned a combined $941m (£603m) before taxes and fees between June 2014 and June 2015".
Jackie Chan came in at number two with $50m (£32m), thanks to being the biggest movie star in China.
1. Robert Downey Jr - $80m
2. Jackie Chan - $50m
3. Vin Diesel - $47m
4. Bradley Cooper - $41.5m
5. Adam Sandler - $41m
6. Tom Cruise - $40m
7. Amitabh Bachchan - $33.5m
8. Salman Khan - $33.5m
9. Akshay Kumar - $32.5m
10. Mark Wahlberg - $32m
Iron Man star Robert Downey Jr has topped Forbes' list of highest-paid actors for the third year running