Messi previously occupied ninth position in 2014 with an estimated worth of $12 million - a drop of one million from the previous year - but does not feature at all in the athletes' top 10 for 2015.
Rory McIlroy and Floyd Mayweather Jr instead enter the top 10 with the undefeated boxing champion having signed several lucrative endorsements before his July fight with Manny Pacquiao.
Madrid star Cristiano Ronaldo, who was seventh a year ago, has also suffered a drop in the rankings and has now moved down to eighth despite releasing a new clothing range.
La Liga rivals Real Madrid and Barcelona had occupied second and third spot in 2014 but have slipped to fifth and seventh respectively, with Manchester United and Bayern Munich also making the top 10.
The New York Yankees lead the way in the team stakes once again, having boosted their brand worth by over $100 million in a year alone from $521 million to $661 million.
Tiger Woods is once more the world's most valuable athlete with estimated worth of $30 million, a $6 million dip on his 2014 estimation, taking the place of NBA star LeBron James ($27 million), who now moves to third.
Another golfing great, Phil Mickelson ($28 million), rises to second while Roger Federer ($27 million) and India cricket captain MS Dhoni ($21 million) round out the individual top five, as they did 12 months ago.
Forbes' top 10 most valuable athletes:
1. Tiger Woods - Worth $30m
2. Phil Mickelson - $28m
3. LeBron James - $27m
- Roger Federer - $27m
5. MS Dhoni - $21m
6. Usain Bolt - $18m
- Kevin Durant - $18m
8. Cristiano Ronaldo - $16m
9. Rory McIlroy - $12m
10. Floyd Mayweather Jr - $11.5m
Forbes top 10 most valuable sports teams:
1. New York Yankees - $661m
2. Los Angeles Lakers - $521m
3. Dallas Cowboys - $497m
4. New England Patriots - $465m
5. Real Madrid - $464m
6. Manchester United - $446m
7. Barcelona - $437m
8. Bayern Munich - $375m
9. Los Angeles Dodgers - $373m
10. New York Knicks - $361m