The victims -- a woman and a man -- boarded the bus in the city's Copacabana beach district early Saturday. Three men later boarded the bus, forced other passengers off, and then raped the woman and took her credit cards, police said.
They handcuffed and beat the man.
Six hours after the assault began, the attackers tossed the victims off the bus in the city of Itaborai, about 50 kilometers (30 miles) away from the start of their trip, the newspaper O Globo reported.
Police said two of the three alleged assailants were arrested. Police did not release the names or nationalities of the victims.
Rio de Janeiro has launched a citywide offensive against crime. Police, backed by army troops, have stormed slums to take them back from drug gangs, CNN said.
The city will host Pope Francis when he attends World Youth Day in July, as well as key matches when Brazil hosts the 2014 World Cup. Rio de Janeiro is also the site of the 2016 Summer Olympics.