President Luiz Inacio Lula da Silva, known as Lula, granted a request for assistance by the state governor.
Ten people were killed in battles between rival drug gangs and police in Rio's largest shanty town, Rocinha, earlier in April.
More than 1,000 police officers were sent to restore the peace.
Tourist fears
According to Brazilian newspapers, Lula decided to grant the requests of Rio governor Rosinha Matheus and the state's public security secretary, Anthony Garotinho, after meeting them earlier this week.
'It's no good having the armed forces preparing for an external war which never comes, when there is a war already going on at home,' is the government's diagnosis
Jornal do Brasil
The troops will take up position in trouble hot-spots in Rio's slums until a permanent police force can be trained to combat the violence.
Several bystanders were among the 10 people killed in shoot-outs in Rocinha earlier this month.
The proximity of the violence to Rio's beaches - the city's main lure for tourists - may have caused particular alarm, commentators suggested at the time.
It was reported earlier this month that the government had decided not to accede to the state's request.
Forces trained
However, despite the government's apparent U-turn on the issue, it had been preparing for the necessity of using troops to maintain order in some of Brazil's most violent slums for some time, reports O Globo.
It says a brigade of 1,800 soldiers has been trained specially to combat urban violence and organised crime.
In addition, the government has reportedly promised to release funds for the construction of two prisons.
Drug-related killings in Brazil's shanty towns have given the country one of the world's highest murder rates.
"'It's no good having the armed forces preparing for an external war which never comes, when there is a war already going on at home', is the government's diagnosis," commented the Jornal do Brasil newspaper.