The children, who were said to be Afghans ranging in age from 10-15, were taken into custody and were being cared for by the city's social services, the BBC reported Saturday.
Railway police started searching the sewer system after receiving reports of children living near railway stations. The children apparently got into the sewers by removing manhole covers, the British network said.
Police said they believe the children arrived as stowaways on trucks arriving from Turkey and Greece.
A charity organization, Save the Children Italy, says more than 1,000 unaccompanied children arrived in Rome last year from Asia, Africa and Latin America, the BBC reported.
Police also recently found illegal immigrants from Bangladesh and China living 20 or more to a room in the city.