Madrid headed a trio of European clubs at the summit of a list which was dominated by NFL franchises.
The nine-times European champions were worth $3.3 billion, up from $1.88bn last year, due in large part to increased revenue from deals with Adidas and Emirates Airlines, Forbes said.
United are in second spot while Spanish champions Barcelona moved up five places to third.
Major League Baseball's New York Yankees fell one spot to fourth, followed by the NFL's Dallas Cowboys and New England Patriots.
Rounding out the top 10 were MLB's Los Angeles Dodgers, the NFL's Washington Redskins and New York Giants, and Arsenal.
With 30 teams in the top 50, the NFL occupies 60 per cent of this year's list. The average value of the top 50 teams is $1.24bn, which is a 16 per cent increase from last year.
The 20 non-NFL teams in the top 50 are comprised of seven each from MLB and football, three from the NBA, two from Formula One and one from the NHL.
The top 10 plus other football/F1 teams – and what Forbes said about them…
2012 - $1.88bn (2nd)
Owned by club members
Los Merengues finished runner-up this year in La Liga and the Copa del Rey, but nab the title of the world's most valuable sports team thanks to the highest revenues in sports ($650 million during the 2011-12 season). Real inked new deals with Adidas and Emirates Airlines over the last 12 months that will bring in roughly $80 million combined annually.
2012 - $2.23bn (1st)
Manager Alex Ferguson retired this year after leading United to its record 20th league title. Sir Alex was at the helm for 27 years and helped create one of the most valuable brands in sports. The team's record shirt sponsorship deal with General Motors for the automaker's Chevrolet brand kicks off in 2014-15 and is worth $559 million over seven years.
2012 - $1.31bn (8th)
Barca added 21-year-old Brazilian star Neymar in June in a transfer deal worth $74 million. Barcelona's jersey sponsorship deal with Qatar Sports Investments is worth an average of $38 million a year through the 2015-16 season.
2012 - $1.85bn (3rd=)
The Yankees sold part of their interest in the YES Network in connection with Fox's purchase of 49% of the regional sport network in late 2012. The team's rights fee from YES will increase from $85 million this season to $350 million in 2042 as part of the deal.
2012 - $1.85bn (3rd=)
The Cowboys have been the NFL's most valuable team since 2007 thanks to the league's highest sponsorship and premium seating revenues - a combined $200 million. The 'Boys inked QB Tony Romo to a six-year, $108 million contract extension in March despite only one playoff win during his eight years under center.
2012 - $1.4bn (6th=)
The Patriots have sold out every game at Gillette Stadium since it opened in 2002 and have more than 60,000 people on a waiting list for season tickets. The Pats increased ticket prices last season for the first time since 2008. Tom Brady and John Elway are the only QBs to start in 5 Super Bowls.
2012 - $1.4bn (6th=)
The Dodgers are close to finalizing a 25-year deal with Time Warner Cable worth as much as $8.5 billion. The deal would kick off next season. The Dodgers opening day payroll in 2012 was $105 million, but this year's is over $200 million thanks to a flurry of trades, contract extensions and free agent signings.
2012 - $1.56bn (5th)
The team traded three first-round draft picks and a second-round choice to the Rams last year in order to select Robert Griffin III (above) with the second overall pick in the 2012 draft. Griffin delivered by leading the 'Skins to their first division title since 1999. His jersey set a single season record for most sales on NFLShop.com.
2012 - $1.3bn (9th)
John Mara, Steven Tisch
The team did not increase ticket prices in 2012, despite heavy demand for tickets following a second Super Bowl title in five years. MetLife is paying more than $425 million over 25 years for naming rights to stadium the Giants share with the Jets.
2012 - $1.29bn (10th)
The Gunners had matchday revenue of $149 million during the 2011-12 season, the third-highest in soccer, due to capacity crowds at 60,400-seat Emirates Stadium.
12. Bayern Munich
2012 - $1.23bn (11th)
Bayern improved on its runner-up finish in the UEFA Champions League last year to win the title in 2013, which was their fifth crown overall. Bayern revenues are twice as high as its nearest competitor, Borussia Dortmund, in the Bundesliga.
2012 - $1.1bn (15th)
Ferrari receives a unique 2.5% share of the annual prize payout for its historical importance to Formula 1. Sponsorship deals with Philip Morris, Shell and Santander are estimated to bring in nearly $250 million annually.
37. AC Milan
2012 - $989m (27th)
2012 - $761m (45th)
Chelsea pocketed a record $76 million for winning the Champions League in 2011-12 while also bagging England's FA Cup. Chelsea won its first UEFA Europa League title this year.
2012 - $800m (40th)
McLaren lost two big pieces in driver Lewis Hamilton, who left for Mercedes this season, and sponsor Vodafone, which announced plans to end its title sponsorship at the end of 2013 after seven years. McLaren will be hard pressed to match the $75 million annual price tag that Vodafone reportedly paid.
Football clubs blew away the competition when it comes to popularity on social media, with Barcelona easily the most popular team in the world.
A combined 60 million following on Twitter and Facebook combined puts the Blaugrana more than 10m ahead of hated rivals Real Madrid, with Manchester United third.
There were five football clubs in the top six, with the LA Lakers far and away the most popular franchise away from the beautiful game.
Chelsea are five times more popular than London rivals Arsenal.
Forbes’ infographic to describe top 50 teams' individual social media following
(Note: Soccer = football and football = NFL)
Infographic by Kai Hecker & Jennifer Sun