The average value of the top 20, which relates to the 2016-17 season, rose by 14 percent over the period.
Real Madrid, who last month won a third straight Champions League title, were second, with a value of $4.08 billion, up 14 percent 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.
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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 $1.69 billion (1.3 billion pounds), a rise it attributed to the decline in the value of the U.S. dollar relative to the Euro.
The 10 most valuable soccer teams (in billions):
1. Manchester United ($4.12)
2. Real Madrid ($4.08)
3. Barcelona ($4.06)
4. Bayern Munich ($3.06)
5. Manchester City ($2.47)
6. Arsenal ($2.23)
7. Chelsea ($2.06)
8. Liverpool ($1.94)
9. Juventus ($1.47)
10. Tottenham Hotspur ($1.23)
(Reporting by Frank Pingue in Toronto,; Editing by Neville Dalton)
