The 20-time English champions won the Europa League and the EFL Cup last season and are now estimated to be worth $3.69bn (£2.9bn) – a 11% increase on the previous year.
United generated revenues of $765m during the 2015/16 season – including $405m from advertising and sponsorships alone – $77m more than both Barcelona and Real Madrid.
The Red Devils are also the most profitable football club on the planet with an operating income of $288m, well above second-placed Real Madrid's $181m income.
Six of the top 10 clubs on the list were Premier League clubs, with Manchester City ($2.08bn) fifth, Arsenal ($1.93bn) sixth, Chelsea ($1.85bn) seventh, Liverpool ($1.49bn) eighth and Tottenham ($1.06bn) 10th.
"The world's most valuable soccer teams are big businesses because of the popularity of their brands and the sport's unparalleled global audience," Forbes said.
"England dominates our list... due to the Premier League having the biggest domestic and international television deals."
Manchester United's value is set to go further up this year due to the club's triumph in the Europa League earning them qualification into next season's Champions League – the richest annual sports event in the world.
"No soccer team has done as good a job as Manchester United at turning a storied history of success on the pitch into profits," Forbes said.
"The team's return to the top spot is a testament to their powerful brand and marketing acumen."
For the fourth straight year, Real Madrid forward Cristiano Ronaldo was named the world's highest-paid football player with earnings of $93m in 2015/16.