About a quarter of all illegal under-age marriages take place in West and Central Africa, a children’s charity says.
Worldwide the number of girls married illegally stands at 7.5 million, a Save the Children study, conducted in conjunction with the World Bank, has found.
It tells the stories of 12-year-olds in Senegal and 15-year-olds in Sierra Leone who are forced to marry.
They tend to fall pregnant quickly and drop out of school.
Some of the young brides come from families where one or both parents have died - they are married off so someone else can support them.
Save the Children says African countries with particularly high rates of illegal marriages are Niger, the Central African Republic and Chad.
The charity calls for all countries that haven't yet done so to set 18 as the minimum age for marriage.
But it says implementing the law is often challenging, so this is just a first step.