"Football Clubs' Valuation: The European Elite" estimated Madrid and United as being worth €2.9 billion at the end of the 2015-16 season.
Barcelona are third, valued at €2.8bn, ahead of Bayern Munich, rated at €2.2bn.
Arsenal (€1.7bn) complete the top five, with the next three places occupied by Manchester City (€1.6bn), Chelsea (€1.5bn) and Liverpool (€1.3bn).
Juventus, worth €983 million, and Paris Saint-Germain (€843m) complete the top 10.
KPMG Head of Sports Network Jean-Luc Leroux said: "It's a study that associates several criteria.
"These include the popularity of clubs with their number of followers on social media, their sporting potential given the value of their players, TV rights and their ownership situation, notably of the stadiums.
"It allows clubs to compare themselves with each other, to see where they are in the European hierarchy and how they progress from one season to another."
PSG are ambitious to move further up the rankings, with L'Equipe reporting that owners Qatar Sports Investments (QSI) hope to increase the club's value to €1.5bn by 2020.
The club was worth just over €100m when QSI took over in 2011, and €540m following the 2014-15 season.
But the stumbling block could prove to be the Parc des Princes which, with a capacity of 48,000, does not provide the scope needed to push the club forward.
QSI plans to increase capacity at the stadium, owned by the City of Paris, to 60,000, but Leroux said: "If they owned their stadium, they would make a leap forward. Without that, it's more complicated."
KPMG reports on all aspects of football finance, including assessing the most successful spenders in the transfer market and an annual overview of trends in football finance.
The Top 10:
Real Madrid (€2.9bn)
Manchester United (€2.9bn)
Bayern Munich (€2.2bn)
Manchester City (€1.6bn)
Juventus (€983 million)
Paris Saint-Germain (€843m)