This is according to a study published by an independent research group, CIES Football Observatory.
It revealed that United had a net spend of €1.075bn since 2012, spending €1.545bn on transfers while making €470million back in player sales.
The Red Devils smashed the Premier League transfer record, when they brought back Paul Pogba for £89.5m in 2016, but the World Cup winner is set to leave as a free agent this summer.
Anthony Martial was signed for £36m in 2015 and Donny van de Beek cost United £40m in 2020. Both players left on loan last month for Sevilla and Everton respectively.
Their city rivals, Manchester City came second with a net spend of €984m, splashing out more than United with €1.699bn but bringing in €715m from sales.
Paris Saint-Germain were third with a net spend of €941m.