The 19-times English champions, who are preparing for a listing on the New York Stock Exchange, were valued at $2.23 billion, up from $1.86 billion last year and 19 percent above No. 2 Real Madrid ($1.88 billion).
All 32 NFL franchises made the top 50 list with the Dallas Cowboys, Washington Redskins, New England Patriots and Super Bowl champion New York Giants all securing spots in the top 10.
The only teams in the top 10 that do not play soccer or American football were Major League Baseball's New York Yankees, who were tied at third with the Cowboys, and the Los Angeles Dodgers, who were tied with the Patriots at number six.
The top 50 list included seven soccer teams, with Arsenal falling three places to 10th overall and Barcelona rising 18 spots to eighth.
Ferrari was the top motor racing team, listed at 15th, while the New York Knicks, in 43rd place, were the lone National Basketball Association franchise to make the list.
Forbes Top 10 teams:
1. Manchester United (soccer) - $2.23 billion
2. Real Madrid (soccer) - $1.88 billion
3. New York Yankees (MLB) - $1.85 billion
3. Dallas Cowboys (NFL) - $1.85 billion
5. Washington Redskins (NFL) - $1.56 billion
6. Los Angeles Dodgers (MLB) - $1.40 billion
6. New England Patriots (NFL) - $1.40 billion
8. Barcelona (soccer) - $1.31 billion
9. New York Giants (NFL) - $1.30 billion
10. Arsenal (soccer) - $1.29 billion
