originally named são sebastião do rio de janeiro, rio was named by the portuguese explorers who discovered it on january 1st, 1502 and, as the area now known as guanabara bay was thought to be a river. 

são sebastião the first was portugal's king at the time of the creation of the city, march 1st 1565, carrying the same name of the saint that was consequently made saint patron of rio.  

in fact, for a long time the city was only called são sebastião, and only later became known as rio de janeiro.

until 1960 rio, also known as cidade maravilhosa (marvelous city), was brazil's capital before the futuristic city of brasilia was declared capital.