The United Nations Children's Fund counted 300,000 unaccompanied and separated children worldwide in 2015 and 2016, up from 66,000 in 2010 and 2011.
Of the 300,000, some 100,000 were caught trying to cross the border from Mexico to the US, UNICEF said.
200,000 applied for asylum in around 80 countries, which included 170,000 lone child refugees who applied for asylum in Europe.
"The number is just the tip of the iceberg," UNICEF’s deputy executive director Justin Forsyth said, as many countries outside of the US and Europe don't have data on numbers of unaccompanied child refugees.
"One child moving alone is too many, and yet today, there are a staggering number of children doing just that - we as adults are failing to protect them," Forsyth said.
The report shows that 92 per cent of children and young people who reached Italy by boat between the beginning of 2016 and end of February 2017 travelled alone, compared to 75 per cent in 2015.
At the G7 Summit in Italy starting May 26, UNICEF is encouraging world leaders to commit to their six-point agenda to protect refugee and migrant children.
The agenda includes protecting unaccompanied children from exploitation, violence and detention, keeping families together and giving children access to education and healthcare.
It also calls for action on underlying causes of large-scale refugee movement and measures to combat xenophobia and discrimination in countries of transit and destination.
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