Paris-based Meridiam, which has teamed up with Germany's Munich Airport as operator for the airport, won a tender last July, offering an annual concession fee of 24.5 million euros (22.32 million pounds) and investment of 608 million euros.
The deal was signed on Wednesday after a Bulgarian court cleared appeals by three other contenders for the concession: a consortium led by France's ADP, Germany's Fraport and Britain's largest airport operator Manchester Airports Group.
Austrian construction company Strabag has a 1% stake in the consortium that will run the airport as well as renovate it and build a third passenger terminal and a new cargo terminal.
Under the deal, the Meridiam consortium has to make an upfront payment of 550 million levs (256 million pounds). The proceeds will help to boost state coffers ahead of a looming economic downturn triggered by the coronavirus pandemic.
The Balkan country picked the Meridiam-led consortium for the deal although it offered a lower amount of investment compared with the other contenders and the third highest annual concession fees.
Bulgarian Prime Minister Boyko Borissov said the deal was done in a transparent way.
"I do hope that after these restrictions over travel because of the pandemic are lifted Sofia Airport will work again as one of the best airports in the region," he said at the signing ceremony where media was not invited because of COVID-19 concerns.
Sofia Airport, which serviced over 7 million passengers last year and handled more than 24,000 tonnes of cargo, has seen a 30% drop in travellers in the first four months of the year due to the COVID-19 pandemic.
Meridiam, with more than $8 billion under management, seven funds and offices across the world, invests in sustainable infrastructure projects. Its airport sector portfolio includes New York's LaGuardia Central Terminal and airports in Madagascar and Jordan.
(Reporting by Tsvetelia Tsolova; Editing by Toby Chopra and Jane Merriman)