JetBlue Airways reached a deal to buy Spirit Airlines on Thursday, a merger that could reshape the airline industry by putting pressure on the nation’s four dominant carriers.

The deal, which values Spirit at $3.8 billion, would create the nation’s fifth-largest airline, with a share of more than 10 percent of the market, behind United Airlines, which has a nearly 14 percent share. Delta Air Lines and Southwest Airlines control more than 17 percent each, while American Airlines has more than 18 percent.

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“We have yet to see an airline merger in the United States in the last 30 years that has been good for consumers, good for labor and good even for the cities and regions in which they operate,” said William J. McGee, a senior fellow for aviation and travel at the American Economic Liberties Project, which pushes for stronger antitrust policies and enforcement.

Spirit and JetBlue’s deal could inspire other airlines to merge to stay competitive, said Helane Becker, a managing director and senior analyst at Cowen, an investment bank. “If this transaction were to get done, it may encourage smaller airlines, especially regional airlines, to consider merging,” she said.

JetBlue and Spirit said they would continue to operate independently, with loyalty programs and customer accounts unchanged, until the merger was complete.