Stashes of cash were found hidden in plastic shopping bags under a bed and in bundles on the floor of the apartment, police confirmed.
Police said the seizure would disrupt "a number of crime groups" and is also believed to be one of the biggest in the country's history.
It followed an operation concentrating on firearms and drug supply across London, which led to three men being sentenced on Friday.
Officers identified large-scale money launderer Sergejs Auzins during the operation, and found he was taking cash to an address in Fulham, West London.
In June 2020, another man, Ruslan Shamsutdinov, was seen struggling to carry heavy bags as he left Porteus Apartments in Fulham.
When officers stopped him, the bags were found containing "a considerable amount of cash", police said.
Officers then searched the apartment to find loads more cash and an organised money laundering system.
More than £5 million was seized from the property and an additional 39,000 Euros and £8,000 was found in Shamsutdinov's home.
A month later, officers arrested a third man, Serwan Ahmadi, who was found with a bag containing almost £60,000 as he left his home in Edmonton, north London.
A search of the property found another £198,000 inside, police said.
Auzins was arrested the next day at his home in Rochester, Kent, where more than £14,000 was recovered - bringing the the total amount of cash seized to nearly £5.4m.
The three men pleaded guilty last year to conspiracy to disguise and remove criminal property.
At Harrow Crown Court on Friday, 36-year-old Shamsutdinov, of Hackney, east London, was jailed for three years and nine months.
Auzins, 46, was sentenced to three years and four months in prison.
Ahmadi, 35, was given a suspended prison sentence of one year and eight months and he was ordered to take part in 150 hours of unpaid work.
Detective Superintendent Jason Prins said: "As part of a long running operation against gun crime and large scale Class A drugs supply across London, over five million pounds in criminal cash was seized from an address in Fulham."
He added: "This is the largest ever single cash seizure by the Met and I believe one of the largest ever nationally.
"Organised crime is motivated by money and is one of the biggest causes of violence on our streets. Tackling this violence is our main priority.
"This seizure, and the conviction of these men for laundering that money, will have a major impact on the ability of a number of crime groups to carry on with their activities."