The Central African Republic has the highest prevalence of child marriage among males, at 28 percent, followed by Nicaragua at 19 percent and Madagascar at 13 percent.
Data from 82 countries showed that boys are being married at high rates around the world, from sub-Saharan Africa to Latin America, South Asia and East Asia, and the Pacific.
"Child grooms are forced to take on adult responsibilities for which they may not be ready, said UNICEF executive director Henrietta Fore.
"Early marriage brings early fatherhood, and with it added pressure to provide for a family, cutting short education and job opportunities."
Worldwide, child marriage disproportionately affects girls over boys.
One in five young women between the ages of 20 and 24 was married before her 18th birthday, compared to one in 30 young men.
UNICEF estimates at 765 million the number of child brides and child grooms worldwide.