Allegiant and Norwegian Air are seen as trailblazers to officials at the Port Authority, because if their service succeeds, it will encourage other airlines to offer flights at Stewart.

“Stewart has enormous potential to be a real gateway airport to the New York area,” said Rick Cotton, the authority’s executive director. The agency has spent $200 million over the past decade to improve the airport, and an additional $30 million has been set aside for the construction of a 20,000-square-foot hall for international arrivals. When complete in 2019, the hall will be able to process 400 passengers an hour.

By focusing on a low-cost carrier identity, Stewart is part of a growing trend among airports in the United States. It is following a pattern established in Europe and Asia, where budget airlines have reinvigorated smaller airports on the outskirts of major cities, said Matthew J. Cornelius, vice president of air policy at Airport Council International, an industry group.

“They’ve had the ability to strengthen their power, and now the Europeans are coming across the Atlantic,” Mr. Cornelius said, adding that the international airlines were seeking smaller airports like Stewart, as well as those in Hartford and Providence, R.I. “They’re used to operating at secondary airports, so it’s a natural.”

This attracts passengers like Gavin Bamford of Northern Ireland, who arranged to visit family in Pennsylvania after finding a $364 round-trip ticket to Stewart Airport on Norwegian Air’s website. Before he returned to Belfast, he said that if fares remained low, he would come back to Stewart, rent a car and drive 400 miles to see relatives in Toronto.

“Stewart was not just about New York City for us,” said Anders Lindstrom, Norwegian Air’s director of communications for North America. “We see a lot of customers from upstate New York who will drive for hours to get there, as well as from northern New Jersey and Connecticut.”