The practice, which often deprives girls of education, is still prevalent in indigenous rural communities.
A year after Guatemala banned child marriage, girls are getting married in rural indigenous communities that are unaware of the ban and see the practice as acceptable, experts say.
Guatemala outlawed child marriage in August 2017, making it illegal for anyone under age 18 to wed under any circumstances.
No data exists to show how many girls have married since then - but anecdotal evidence gathered by groups working with rural indigenous communities suggests the practice persists.
Nearly a third of girls in Guatemala are currently married by 18 and many girls live with partners in informal unions.
Many of Guatemala's poor Mayan indigenous communities, where child marriage is most common, remain unaware of the outright ban, experts and advocates say.
Education in Guatemala
More than two million children in Guatemala are out of school, mostly in rural areas. By 17 years old, only 26% of indigenous girls are still at school. By 18, almost 40% of indigenous Mayan girls are married.
Source: Girl Up Picture: Plan International
"What we've been hearing in the communities is that not much has changed since the law was passed," said Emma Puig, head of gender equality in Latin America at the children's charity Plan International.
"The big challenge is working on changing mindsets, social norms that find it normal to see a girl under 18 living with a man who most of the time could be her grandfather," she told the Thomson Reuters Foundation.
Driven by poverty and cultural traditions in a country with one of the highest rates of child malnutrition in Latin America, indigenous girls are often married off so families have one less mouth to feed.
According to rights group Women's Justice Initiative (WJI), in the village of Chuiquel alone in western Guatemala at least 30 girls, most aged 15 and 16, have been reported to be in informal unions since the marriage ban.
WJI hosts workshops in isolated communities led by local Mayan women who spread the message about the marriage ban and educate girls about their rights.
"It's key for girls to know they have other options and can do other things than just thinking their only option is to get married and have children," said Viviana Patal, a lawyer at WJI.
"The impact of the workshops among families has been to question and rethink the importance of study and sending their daughters to school, the idea that going to school can be a tool to having a better quality of life."
While rates of child marriage have been slowly declining worldwide, each year 12 million girls are married before age 18, according to campaign group Girls Not Brides.
Latin America is the only region not to have seen a significant drop in child marriage in the past three decades, with the highest prevalence in Nicaragua, the Dominican Republic and Brazil.
Most Latin American countries ban marriage until age 18 - but many make exceptions with permission of parents or a judge.
Getting boys and young men to question traditional gender roles and what it means to be a man is key to the solution, Patal said.
"Young men have social pressures too. They feel if I don't have a wife and children then I'm not a man," Patal said.
Child marriage typically deprives girls of education, keeps them in poverty and puts them at risk of domestic and sexual violence, experts say.
"Girls are trapped in a life of violence for the rest of the lives," said Alejandra Colom, Guatemala country director for the Population Council, a US-based research charity.
An under-age bride's first pregnancy is in fact the product of rape but "this is rape that is condoned because it is in a union that is accepted by the community", she said.