The Red Devils dropped from second in the 2018 list to sixth as Real (third) and Barca (fourth) both retained their positions achieved last year.
It is the fourth successive year that the Cowboys have topped the list - with a value of $5 billion - more than iconic MLB team the Yankees ($4.6b) and Real ($4.24b) and Barca ($4.02b), with the NBA's New York Knicks ($4b) rounding out the top five.
Of the 50 highest-rated sports TV broadcasts last year in the U.S., nine were regular-season Cowboys games. The team's popularity helps owner Jerry Jones bring in an estimated $340 million in sponsorship and seating revenue at AT&T Stadium, which is twice as much as any other team, Forbes noted.
"On and off the field, in season and out of season, there is a small soap opera going on every day," Jones told Forbes last year. "Everyone knows that marketing, especially in this day and time, is just another way to promote the circus, so to speak."
The NFL as a whole is still the most dominant sports league as more than half of the top 50 are football teams.
The NBA has also increased its value as the Knicks headline nine basketball teams on the list this year, ahead of the Los Angeles Lakers ($3.7b) and the Golden State Warriors ($3.5b).
Forbes' 50 most valuable sports teams:
1. Dallas Cowboys ($5b)
2. New York Yankees ($4.6b)
3. Real Madrid ($4.24b)
4. Barcelona ($4.02b)
5. New York Knicks ($4b)
6. Manchester United ($3.81b)
7. New England Patriots ($3.8b)
8. Los Angeles Lakers ($3.7b)
9. Golden State Warriors ($3.5b)
10. Los Angeles Dodgers/New York Giants ($3.3b)
12. Boston Red Sox/Los Angeles Rams ($3.2b)
14. Washington Redskins/Chicago Cubs ($3.1b)
16. San Francisco 49ers ($3.05b)
17. Bayern Munich ($3.02b)
18. San Francisco Giants ($3b)
19. Chicago Bulls/Chicago Bears ($2.9b)
21. New York Jets ($2.85b)
22. Houston Texans/Boston Celtics ($2.8b)
24. Philadelphia Eagles ($2.75b)
25. Manchester City ($2.69b)
26. Denver Broncos ($2.65b)
27. Green Bay Packers ($2.63b)
28. Atlanta Falcons ($2.6b)
29. Baltimore Ravens ($2.59b)
30. Pittsburgh Steelers ($2.59b)
31. Seattle Seahawks ($2.58b)
32. Chelsea ($2.58b)
33. Miami Dolphins ($2.58b)
34. Oakland Raiders ($2.42b)
35. Minnesota Vikings ($2.4b)
36. Indianapolis Colts ($2.38b)
37. Brooklyn Nets ($2.35b)
38. Houston Rockets/Carolina Panthers/New York Mets ($2.3b)
41. Los Angeles Chargers ($2.28b)
42. Arsenal ($2.27b)
43. Dallas Mavericks ($2.25b)
44. Los Angeles Clippers ($2.2b)
45. Liverpool ($2.18b)
46. Arizona Cardinals ($2.15b)
47. St Louis Cardinals/Kansas City Chiefs ($2.1b)
49. Jacksonville Jaguars ($2.08b)
50. New Orleans Saints ($2.08b)