State Television showed an excavation team removing vases from the tomb, which resembled a deep well.
"I am convinced that the excavations will bring more tombs to light," ANSA quoted Rome's archaeology commissioner, Eugenio La Rocca, as saying.
Archaeologists were excavating under the level of the ancient forum, a popular tourist site, when they dug up the tomb, which they suspect is part of an entire necropolis, Italian news agency ANSA reported.
Inside it was a perfectly preserved funeral urn dating from a thousand years before Christ, the city's cultural superintendent said.
"We have also found burned human bones and traces of animal bones, perhaps the remains of a funerary meal, as well as small bronze objects that we still have to examine," Mr La Rocca said.
State Television quoted experts as saying the tomb appeared to date to about 1000 BC, meaning the people who constructed the necropolis pre-dated the ancient Romans by hundreds of years.
It dates from the transition of the ice and bronze ages, said Mr La Rocca.
The tomb and its artefacts, which date from the 11th century BC, will help historians learn more about a time before the foundation of Rome around 750 BC.
Legend has it that Rome was founded in 753 BC by Romulus and Remus, the twin sons of the god of war, Mars.
Last year, archaeologists who have been digging for some twenty years in the forum said they believed they found evidence of a royal palace roughly dating to the period of the legendary founding.