With its impact expected to intensify in the next six months, children in Somalia, Kenya, Zimbabwe, Ethiopia and Malawi are particularly at risk, the charity said.
Unicef called on governments to step up their aid plans before the situation deteriorates or face “stark pictures in the media” of starving children.
The warning comes ahead of a meeting of world leaders in Paris to discuss targets for slowing climate change and weather-related events, and as European leaders meet in Malta to discuss the refugee crisis in Europe.