"The number of cases has doubled in the past two weeks. It is extremely worrying," a UNICEF spokesman said. Some 30 percent of those affected are under 14.
Cholera is endemic on the volcanic island in the Gulf of Guinea off West Africa.
An outbreak in May claimed three lives among 255 people hit by the disease, which is spread through contaminated water and food, causing severe dehydration due to diarrhoea.
UNICEF, which is supplying clean water to affected areas and latrines, is also considering a government request to deploy its own health experts in hospitals due to the limited number of staff doctors, the spokesman said.
It has also helped to launch a radio campaign broadcasting simple prevention measures for the population to take, such as boiling water before use.
Nearly a fifth of Sao Tome's people have no access to safe drinking water and just 9 percent use hygienic toilet facilities, according to UNICEF.