Cohabiting couples in Burundi have until the end of the year to get married or face legal consequences.
The government order comes after President Pierre Nkurunziza launched a campaign “to moralise society”.
A government spokesman said a crackdown on informal relationships was needed to combat a population explosion.
He said too many schoolgirls were getting pregnant and men were taking advantage of women by cohabiting with several simultaneously.
Burundi has been in crisis since 2015 when Mr Nkurunziza, a born-again Christian, announced he would run for a controversial third term.