Officials say 54,000 families living in an area prone to extreme weather are in a critical situation.
So far this year, some 25 children are believed to have died of hunger.
The UN's World Food Programme (WFP) announced it would start distributing 20 tonnes of nutritional biscuits to the worst affected areas.
President Colom made his announcement on Tuesday, saying the declaration of a state of public calamity would help the government to access the funding and resources needed to tackle the food crisis.
"There is food, what is lacking is the money for the affected people to buy food," Mr Colom said. "We are not going to wait until we've reached starvation levels to act."
The move allows the government to make emergency purchases of food.