The English Premier League runners-up, who last year ended European champions Real Madrid's four-year stint at the top of the list, were valued at US$4.12 billion, up 12% from a year ago, Forbes said in a statement.
The average value of the top 20, which relates to the 2016-17 season, rose by 14% over the period.
Real Madrid, who last month won a third straight Champions League title, were second, with a value of US$4.08 billion, up 14% from the previous year.
Barcelona (3 billion pounds), Bayern Munich (2.3 billion pounds) and United's neighbours Manchester City (1.8 billion pounds) rounded off the top five.
Six English clubs feature in the top 10.
According to Forbes, which bases valuation on the level of equity plus net debt, Barcelona or Real Madrid could soon take over top spot - the former because of their consistently high performance in the Champions League and the latter because of additional expected revenue from their renovated stadium when completed in four years.
Forbes said the 20 most valuable soccer teams are now worth an average of US$1.69 billion (1.3 billion pounds), a rise it attributed to the decline in the value of the US dollar relative to the Euro.
The 10 most valuable soccer teams (in billions):
1. Manchester United (US$4.12)
2. Real Madrid (US$4.08)
3. Barcelona (US$4.06)
4. Bayern Munich (US$3.06)
5. Manchester City (US$2.47)
6. Arsenal (US$2.23)
7. Chelsea (US$2.06)
8. Liverpool (US$1.94)
9. Juventus (US$1.47)
10. Tottenham Hotspur (US$1.23)
