The Cloaca Maxima was one of the world's earliest sewage systems. Constructed in ancient Rome in order to drain local marshes and remove the waste of one of the world's most populous cities, it carried effluent to the River Tiber, which ran beside the city.

The name literally means Great Sewer. It is believed to have been initially constructed around 510 BC under the orders of the last king of Rome, Tarquin II.

This public work was largely achieved through the use of Etruscan engineers and large amounts of semi-forced labour from the poorer classes of Roman citizens.

Although Livy describes it as being tunnelled out beneath Rome, he was writing a great deal after the event. From other writings and from the path that it takes, it seems more likely that it was originally an open drain, formed from streams from three of the neighbouring hills, that were channeled through the main Forum and then on to the Tiber. This open drain would then have been gradually built over, as building space within the city became more valuable. It is possible that both theories are correct, and certainly some of the lower parts of the system suggest that they would have been below ground level even at the time of the supposed construction.

There were many branches off from the main sewer, but all seem to be 'official' drains that would have served public toilets, bath-houses and other public buildings. Private residences in Rome, even of the rich, would have relied on some sort of cess-pit arrangement for sewage.

The Cloaca Maxima was well maintained throughout the life of the Roman Empire and there is evidence to suggest it was still working long after the traditional fall of the Western Empire. In 33 BC it is known to have received an inspection and overhaul from Agrippa, and archaeology reveals several building styles and material from various ages, suggesting that the systems received regular attention. In more recent times, the remaining passages have been connected to the modern-day sewage system, mainly to cope with problems of backwash from the river.

The Cloaca Maxima was thought to be presided over by the goddess Cloacina.

The body of Saint Sebastian is said to have been dumped in the Cloaca Maxima, which is the subject of a well-known artwork by Lodovico Carracci.