The vote affects about 14,500 agents who work at both American and US Airways. The workers will be represented jointly by the Communications Workers of America and the International Brotherhood of Teamsters, which represented agents at US Airways for more than a decade prior to its merger with American last year to create the world's largest airline by passenger traffic.

The CWA said 9,640 agents voted in favor of the union while another 1,547 opposed it.

Separately, United Airlines said it ratified a new labor agreement with the airline's 48 maintenance instructors, represented by the International Association of Machinists and Aerospace Workers.

The National Mediation Board has also ruled that the Allied Pilots Association, which represents pilots at American, will bargain on behalf of US Airways' pilots in future contract negotiations, according to a spokesperson.

The decisions underscore a recent string of votes to unionize at U.S. airlines amid improving profits.

American's vote could spur more organizing efforts in the U.S. South, where most of the American agents are based, union leaders said.

The vote also marks a change in mindset at American, which discouraged attempts by agents to organize over the past 19 years, according to union leaders and industry experts.

American's new management has "a positive attitude about what employees decide to do," said CWA President Larry Cohen.

Three-quarters of the passenger service agents work in the U.S. South, making the pro-union vote rare for the region, Cohen said.

AFL-CIO President Richard Trumka, in a statement, added, "this victory will energize ongoing organizing efforts in the South."

Industry consultant Robert Mann said Doug Parker, the new American group chief executive, was "attempting to build a team, not to divide interests."

American and US Airways still face integration issues from their December 2013 merger. Pilot unions have yet to agree on their relative seniority at the two airlines. Many other workers - including ticketing and gate agents - continue to operate separately.

Cohen said he anticipates bargaining for a revised contract will begin in November. US Airways ticket and gate agents, numbering about 5,500, are represented under the union's existing contract, whereas American's 9,000 agents are not.

Employees expect a return to more generous terms that had prevailed before airlines announced cuts during their struggles with bankruptcy. But Cohen added that "people want to keep American (profitable) as well."

American's stock rose 91 cents, or about 2.45 percent, to close at $37.98 on Tuesday.