The AU is holding a two-day conference in Zambia's capital, Lusaka, to review the progress being made and to persuade governments to make the practice illegal as it says it robs young girls of their future.
"The sheer number of girls affected - and what this means in terms of lost childhoods and shattered futures - underline the urgency of banning the practice of child marriage once and for all," Unicef executive director Anthony Lake said.
Unicef says the poorest families on the continent often see marriage rather than education as the best way to secure a girl's future and protect her chastity.
The BBC's Nomsa Maseko in Lusaka says achieving this requires an uphill task of convincing local traditional leaders and parents that keeping girls in school and out of marriage is the way forward to break the cycle of poverty.The report says the percentage of young girls in Africa who are married before the age of 18 has fallen from 44% in 1990 to 34% today.
But massive population growth means there will still be an increase in the number under-age marriages unless far more ambitious action is taken, it added.
Africa's population of girls under 18 is predicted to rise from 275 million today - 25% of the global total - to 465 million by 2050, 38% of the total.