Aerospace giant Bombardier, which employs around 4,000 people in Belfast, has won a deal worth up to $467 million (£373m) to sell 10 of its regional jets to a Dublin carrier.
CityJet has signed a conditional purchase agreement for six CRJ900 aircraft, and has taken options on another four.
The conditional offer is worth around $280m (£224m) - but if CityJet exercises all its options, the value of the deal could increase to $467m.
And CityJet said it will use the planes for contract flying with Scandinavian Airlines. Belfast staff work on the design and manufacture of the centre fuselage and nacelles for the CRJ regional jets.
Ryan DeBrusk of Bombardier said: "CityJet has become one of our largest European CRJ900 aircraft advocates in a short period of time and we are delighted that they continue to put their confidence in Bombardier and its products.
"The CRJ900 aircraft is ideally suited to growing markets across Europe and is creating excellent value for a wide variety of operators with its superior performance, economics and enhanced cabin amenities."
Pat Byrne, executive chairman of CityJet, said: "The reputable, modern CRJ900 aircraft offers great passenger comfort while maintaining cost effectiveness and operational flexibility - both important factors in our business model
"We have been very satisfied with the performance of the CRJ900 regional jets which have proven themselves to be the ideal and most efficient aircraft for our contract flying with Scandinavian Airlines."
CityJet already owns a fleet of eight CRJ900 aircraft, used for contract flying with Scandinavian Airlines.
The airline will also take delivery of four new CRJ900 aircraft in early 2017 as part of a deal announced last year, bringing CityJet's owned fleet of CRJ900 aircraft to 12.
CityJet is based in Swords, Dublin. The airline was established in 1993 to operate services between Dublin and London City Airport and today operates 10 routes from across Europe to London City, where it currently holds approximately 30% of the airport slots.
CityJet also has a large contract flying business - known as 'wet lease' - including services on behalf of Air France as well as Scandinavian Airlines, and operates a wide range of charters on behalf of sports groups, the entertainment industry, tour operators and corporate customers.
In March last year it was bought up by a an investment group led by Pat Byrne.
Bombardier said its CRJ Series is "the world's most successful regional jet programme".
The jets are optimised for medium-haul regional routes and account for over 20% of all jet departures in North America. The company has firm orders for just over 1,900 of the CRJ Series aircraft, which also includes the CRJ700 and CRJ1000. Last year Bombardier announced plans to cut 7,500 jobs worldwide, including 1,000 in Northern Ireland.