The deal - with one of the airport's existing low-cost carriers - was finalised as Belfast hosted a major airline conference.
It is believed will also create 50 new jobs. Details will be announced on Wednesday.
About 1,200 people are attending the Routes Europe conference at the Waterfront Hall.
Speaking at the event, airport chief executive Graham Keddie said: "We shook hands on it at the weekend."
The three-day conference began on Sunday and brings together airline executives.
Organisers believe it is "a brilliant opportunity" to showcase Northern Ireland's tourism sector.
Ryanair, easyJet, Lufthansa, Air France and British Airways are among the many airlines taking part.
In 2016, a record 7.8m passengers used Northern Ireland's airports.
Growth has largely been driven by Ryanair who began operating from Belfast International last year.
But expansion of long-haul routes has been challenging against the dominance of Dublin Airport.
Around Â£1m is being spent hosting Routes Europe, with about half the money coming through Invest NI.
Steven Small, brand director of Routes, said: "Northern Ireland is a business and tourism destination on the rise.
"The demand for travel is increasing so there are significant route development opportunities for Europe's airlines."
Six new European routes are being added to flight schedules at Belfast International Airport.