The law criminalises the marriage of girls under the age of 18. It also prescribes prison sentences of up to 15 years for offenders.
A third of all girls in the West African country are married before their 18th birthday, according to UNICEF.
The country is home to 800,000 child brides, 400,000 of whom were married before the age of 15, the UN agency said.
Often because of poverty, many girls in Sierra Leone do not stay in school. They are then married off by their families to improve their financial situation or to pay off debts.