Monk Bellana Panniyaloka was found guilty of grave sexual abuse of a 16-year-old girl in 2001 and was given the maximum 20-year penalty, court official N.K. Siripala said.
Soon after the sentence, Panniyaloka drank from a bottle he had concealed in his robes and collapsed, Siripala said. He was rushed to a hospital.
"The priest is in critical condition," said Hector Weerasinghe, director of the Colombo National Hospital. He said the 55-year-old monk had consumed insecticide.
The priest belongs to a temple in Nugegoda, just outside the capital Colombo, where the victim was a Sunday school student, Siripala said.
Monday's conviction came a day after a senior Buddhist monk was arrested on charges of sexually abusing a teenager in another Sri Lankan temple, officials said.
The monk, who heads a temple in the coastal town of Matara, 80 miles south of Colombo, allegedly abused a 15-year-old girl, said Dimutu Galappatti, spokesman for the National Child Protection Authority.
Reports of child abuse by Buddhist priests have increased in recent years, but Panniyaloka's trial is the first conviction of a monk on abuse charges.
About 70 percent of Sri Lanka's 19 million people are Buddhists, and monks hold a tremendous influence in this Buddhist-majority country. They play a major role in molding the island's political and social fabric.