MEXICO CITY — A former labor minister and novelist from the governing party, Carlos Alvarado Quesada, was decisively elected president of Costa Rica in a runoff on Sunday, holding off an evangelical Christian singer Fabricio Alvarado Muñoz, who had built an upstart campaign in part on his opposition to same-sex marriage.

The overwhelming victory defied polls in recent weeks that showed the candidates locked in a statistical dead heat or Mr. Alvarado Muñoz with a lead.

“Costa Rica once again delivered a beautiful democratic message,” Mr. Alvarado Quesada, 38, told supporters late Sunday night. “Well done, Costa Rica.” He added: “What unites us is much greater than what divides us!”

The complexion of the contest changed dramatically in January when the Inter-American Court of Human Rights ruled that its signatory nations, including Costa Rica, must legalize same-sex marriage. The court was established by the Organization of American States.

Some countries in the region — including Argentina, Brazil, Colombia and Uruguay — have legalized same-sex marriage, though many others have not.

While the Solís administration welcomed the decision and vowed to comply with it, the court’s opinion set off a conservative backlash in Costa Rica, a traditionally Roman Catholic country. A poll by a research center at the University of Costa Rica found that a majority of respondents who knew about the ruling opposed it.