Forbes now values the Spanish giant at $3.3bn (£2.2bn), with Man Utd in second place on $3.17bn. Barcelona is third.
Forbes said Real Madrid's financial success came despite the continuing woes of the Spanish economy.
"Real Madrid has been consistently delivering the highest revenues of any soccer team in the world, despite an intense economic meltdown in Spain," said Forbes.
"With superstars like Cristiano Ronaldo on the pitch, and coach Jose Mourinho on the bench, Real Madrid has thrived under the leadership of billionaire president Florentino Perez."
In coming to its figures, Forbes looks at a football club's various revenue streams, such as television money, ticket sales, merchandise and concessions.
After Barcelona on $2.6bn, Forbes found that Arsenal was the fourth richest club, worth $1.33bn, followed by Bayern Munich ($1.31bn), AC Milan ($945m), Chelsea ($901m), Juventus ($694m), Manchester City ($689m) and Liverpool ($651m)
David Beckham, now with French team Paris St-Germain, was the highest earning player, followed by Real Madrid's Ronaldo.
