Patrice Faye, 58, was accused of raping five girls aged 13 to 17 -- students at a school he set up for poor children. He denied the charges.
Supreme Court spokesman Elie Ntungwanayo said Faye, a long-time resident, could appeal the sentence.
The trial had been held behind closed doors since June 16. Prosecutors had sought a 35-year prison term.
Faye was arrested in early April and was detained in a prison in central Bujumbura following accusations by the girls.
In a letter addressed to AFP in May, Faye said a doctor had confirmed that three of his accusers were still virgins.
"My client is very shocked. This is someone who has spent 35 years of his life in Burundi doing good things and he is being convicted because of young girls who want to extort money," his lawyer Fabien Segatwa said.
"Patrice Faye did nothing. He still maintains his innocence," said the lawyer, adding: "It is a death sentence given his age."