The thieves rappeled 12 metres (40 feet) to the ground after entering via the roof, dodging movement sensors to spend hours rummaging through thousands of works destined for an auction in the US.
They left by the same route with a haul whose total value was estimated at around two million euros ($2.4 million).
Police arrested four Romanian suspects in June 2019 when they raided around 30 properties in the country's northeast, while around 10 others -- whose nationalities were not revealed -- were arrested in Britain.
But EU agency Eurojust said the January arrest in Turin of the suspected ringleader -- also Romanian -- who cooperated with authorities was "decisive" for recovering the works.
The thieves' loot was discovered in a house in Romania's northeastern Neamt county, the prosecutors said.
London's Metropolitan police said in a statement that the "irreplaceable" works had been "buried underground", posting a picture on its website of a hidden compartment under a house.
They had been stolen by an organised crime group that "flies members into the UK to commit specific offences and then fly them out of the country shortly afterwards," the Met said.
It is "linked to a number of prominent Romanian crime families who form part of the Clamparu crime group" with "a history of complex and large-scale high value thefts", the statement added.
"This operation is a double success for law enforcement who tracked the suspects down and recovered the stolen treasures before they went for sale," Eurojust said.