A WFP spokesman said villagers in Niger described the situation as worse than 2005, when thousands died of hunger.
After a prolonged drought, heavy rains have now hit parts of the country, killing at least six people.
The WFP says 17% of children, or one in five, are acutely malnourished.
The figure – based on national surveys in May and June – is well above the WFP’s normal 15% threshold for declaring an emergency.
It is appealing for $213m (£136m) in aid, but is still 40% short of its target, a spokesman said.