Australians are being urged to use the day - October 11 - to reflect on outdated cultural norms, values and gender stereotypes that contribute to inequality.
Around the world, 12 million girls under the age of 18 are married each year, which equates to one almost every three seconds.
While early and child marriage affects both girls and boys, the impact on girls is much greater and those who grow up in poverty or living through a crisis, conflict or disaster are particularly at risk.
For Akec Makur Chuot, the topic is deeply personal.
The former AFLW player grew up in a Kenyan refugee camp, where she lived for 12 years before settling in Perth.
Her sister, who remained in Kenya, was forced into marriage at 15 to a much older man.
"People think it's a foreign thing that happens overseas, but it's happening in our community," Makur Chuot said.
"It happened in South Sudan, but I was obviously in Australia, and it affected me because I knew that (my sister) was a young girl who had a lot of dreams, just like millions of girls who have been forcibly married off at such a young age."
International Day of the Girl, celebrated on October 11, aims to spotlight the potential of girls around the world and the barriers they can face to freedom, safety, justice, health and education.
This year's theme brings attention to child marriage, which remains widespread despite large-scale policy action and legislative reform.
A report by Plan International Australia surveyed more than 250 girls aged 15 to 24 who married as children, as well as activists working to eradicate child marriage.
The study, conducted across 15 nations, found 66 per cent of child brides had dropped out of school and 75 per cent were unemployed.
"Something that really struck me is the girls we spoke to talked about being very isolated," Plan International Australia chief executive Susanne Legena told AAP.
"They miss their friends. They're young girls (and) they're living in societies and cultures where the only status you have as a woman is to be married or to have children."
While child marriage was illegal in Australia, Ms Legena said the nation's girls still faced other forms of gender inequality.
"The norms, the values, those gender stereotypes, the dismissive views of girls and what they can be, they exist in Australia," she said.
"We have to tackle that attitude here as well. It doesn't manifest in child marriage, but it does manifest in sexual harassment, gender inequality and gender-based violence."
Governor-General Sam Mostyn, recently announced as a new patron for Plan International Australia, said International Day of the Girl was a time to celebrate and highlight the change girls made around the world when they were not held back.
"If girls could be educated, empowered and included, it would mean a better world, not just for girls, but a better world for everyone," she said.
"Here in Australia, this is a moment of reckoning ... we cannot stop until we are all equal."
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