One girl under 15 falls victim to child marriage every seven seconds, according to a new study.
Save the Children found that girls as young as 10 are being married off to men - often much older than themselves - in countries such as Afghanistan, Yemen, India and Somalia.
India has the largest number of child marriages in the world, with 47% of girls married under 18 (24.6 million), the report stated.
According to current rates, the total number of women married in childhood is expected to grow to 950 million by 2030.
Save the Children's "Every Last Girl: Free to live, free to learn, free from harm" ranked countries according to child marriage, schooling, teen pregnancy, maternal deaths and number of female MPs.
Countries at the bottom of the list included Niger, Chad, Central African Republic, Mali and Somalia.
At the top included Sweden, Finland, Norway, Netherlands and Belgium. The UK ranked 15th out of 144 countries.
Girls from poor families are more likely to be married early than their richer peers, the report found.
Meanwhile, the report stated that most countries are struggling to achieve gender parity amongst MPs, regardless of the size of their economy.
Save the Children's CEO Kevin Watkins said: "Child marriage isn't just a form of discrimination, it's a form of violence.
"Forcing girls to marry much older men robs them of their freedom and amounts to sexual slavery.
"Instead of being in school, married girls face domestic violence, abuse and rape.
"They fall pregnant and are exposed to STIs including HIV. Bearing children before their bodies are ready means girls suffer complicated deliveries and even death".
The international community has pledged to end child marriage by 2030,but if current trends continue the number will grow to 1.2 billion by 2050.