Having recognised the need to overhaul a squad which finished a lowly seventh in the Premier League last season, Louis van Gaal invested £157.7m million to bring Angel Di Maria, Marcos Rojo, Ander Herrera, Luke Shaw, Daley Blind and Radamel Falcao (on a loan deal) to the club.
Barcelona are second to United when it comes to spending after splashing the cash on Luis Suarez and Co.
The Blaugrana have had a busy close season, signing Claudio Bravo and Marc-Andre ter Stegen in goal, Jeremy Mathieu, Douglas and Thomas Vermaelen in defence, Ivan Rakitic in midfield and the Uruguayan up front.
Their total summer spending is €174.8 million [£138.5m], placing them just ahead of Liverpool. The Reds got a big £70m pay-off from the sale of Suarez and their return to the Champions League and have been ambitious throughout the summer.
Adam Lallana, Dejan Lovren and Rickie Lambert all joined from Southampton, Lazar Markovic was a big-money acquisition from Benfica and Emre Can arrived from Bayer Leverkusen, all before the £16m signing of Mario Balotelli.
Their spree totalled £133.6m, comfortably putting them second in Premier League spending behind United.
Real Madrid are fourth in the list after spending €136.4m [£108m]. The Spanish giants have had a reasonably busy summer, signing Keylor Navas, Toni Kroos and James Rodriguez - the latter for a whopping €80m [£63m] - as well as Javier Hernandez on loan.
Hot on their heels are city rivals Atletico Madrid. The Liga champions lost key members of their squad this summer but have been ambitious in the market, signing Mario Mandzukic, Antoine Griezmann, Alessio Cerci and more for a combined €123.7m [£98m].
|SUMMER SALES | Top 10 clubs by expenditure
|Gross outlay||Net outlay|
One of the teams to have raided the Vicente Calderon for new talent was Chelsea, who signed Diego Costa and Filipe Luis from the Spanish capital, along with Cesc Fabregas from Barcelona and Loic Remy from QPR to take their summer spending to £94.1m.
Staying in London, Arsenal are sixth after buying the likes of Alexis Sanchez, Calum Chambers, Mathieu Debuchy, David Ospina and, at the last minute, Danny Welbeck for just over a total of £85m.
Southampton have seen many players leave this summer, but have been active themselves by signing Dusan Tadic, Shane Long, Graziano Pelle, Toby Alderweireld, Sadio Mane and more for more than £67m, but they are the only side in our table to have recorded a profit thanks to their sales.
Manchester City have had their spending curtailed by Financial Fair Play but have still splashed out £57.7m on new players, the majority of it going on Eliaquim Mangala and Fernando from Porto.
The final name in the top 10 is Roma, who spent big on Juan Iturbe and Kostas Manolas among others to take their total over the £50m mark - the highest by far in Italy.
Interestingly, all but one of the top 10 are English and Spanish sides, with Germany's Borussia Dortmund the 11th highest spenders, just ahead of Bayern Munich.
*Numbers via Transfermarkt.