During a police probe into allegations that the girl had been raped by her stepfather, investigators uncovered evidence that she had had consensual sex with another man.
"Though she has been sentenced she will be lashed once she turns 18," the court official, who asked not to be named, said. "But the sentence will be enforced immediately if she wants it to be carried out now."
Premarital sex is illegal under the Indian Ocean nation's strict Islamic law.
The child's stepfather faces up to 25 years in prison if convicted of rape and a murder charge, after he allegedly killed a baby which resulted from his rape of his step-daughter.
The premarital sex charge against the teenager has been condemned by rights groups and the government has urged leniency towards the girl, saying she had been traumatised by the repeated rapes by her stepfather.
President Mohamed Waheed's spokesman Masood Imad said that the girl should be treated as a victim rather than a perpetrator of a crime.
It was unclear if the girl's male partner would also be charged with having premarital sex.