Koirala received 405 of 553 votes, speaker Surya Bahadur Thapa said.
"We congratulate respected member Sushil Koirala for securing majority votes in the prime minister's election," Thapa said. "Now the parties should focus on writing the constitution within a year."
Koirala, the lone candidate for the post, represented the country's second-largest party, the Community Party, Nepalnews.com reported.
As part of his campaign, Koirala promised to complete the constitution within a year and implement past agreements of the peace process. He also said he'd hold local elections within the next six months, the news website said.