Oscar Niemeyer, the Brazilian architect who designed the United Nations building and his country's futuristic capital of Brasília, died on Wednesday in a hospital in Rio de Janeiro. He was 104.

Mr. Niemeyer became one of the 20th century's most important architects by infusing modernism with the tropical sensibilities of his native Brazil. He also attracted fierce critics—including many people who worked in his Brazilian buildings and found them unsuitably hot for the climate.

Mr....