The Hospital Samaritano said in a Wednesday statement that the 104-year-old was admitted on Tuesday and is in stable condition following the insertion of a gastric tube.
He is "lucid and breathing without the aid of machines," the statement said. No release date has yet been set.
Niemeyer spent two weeks at the Samaritano last month, also for dehydration. He was also hospitalized there in May for pneumonia and dehydration and last year for a urinary infection.
Niemeyer designed much of Brazil's futuristic capital, Brasilia, as well as Rio's Sambadrome, where the city's annual carnival parade is held. He also helped design the United Nations building in New York City.