Altinum (in Altino, a frazione of Quarto d'Altino) was an ancient town of the Veneti 15 km southeast of modern Treviso, close to the mainland shore of the Lagoon of Venice. It was also close to the mouths of the rivers Dese, Zero and Sile. A flourishing port and trading centre during the Roman period, it was destroyed by Attila the Hun in 452. The town recovered, but was later abandoned when sea-borne sand began to cover it over. Its inhabitants moved to Torcello and other islands of the northern part of the lagoon.

Today Altinum is an archaeological area and has a national archaeological museum.

Altinum was a Venetic settlement. The earliest human presence in the area is dated to the 10th century BCE and is related to hunter-gatherer groups. The earliest evidence of a settlement nucleus is dated from the mid-8th century to the mid-7th century BCE. In the 7th century BCE the settlement moved slightly to the northwest, in its historical location. A sacred area which is dated to the late 6th century BCE and developed in the 5th and 4th centuries BCE had votive offering objects from Greek, Magna Graecian, Etruscan and Celtic areas. This suggests that Altinum was the main port of the Veneti in the proto-historical age.

Archaeology indicates that in the late 6th/early 5th century BC, the Veneti had precocious contact with Celtic areas through the communication routes along the rivers Adige and Piave in the major centres in the plain of the River Po (Este and Padua and the Adriatic ports such as Atria, modern Adria, and Altinum). There was a trading relationship between high-ranking families. There was a degree of intermarriage. Some Celts settled in the region. Celtic gift and fashion objects appear in the burials of important families.

In the early 4th century BCE, the Gauls invaded the plain of the Po as far as Verona. This led to a gradual ethnic mixing and loss of cultural identity, especially in the border areas of the Veneti: Verona in the west and the Lagoon of Venice and the Piave valley in the east. In some places there was at times a transition from the traditional practice of cremation to inhumation and the deposition of weapons in burials, which was an exception to Veneti funerary culture. An example of this was found in a cemetery in Altinum. Such evidence suggests that inter-ethnic relations went beyond just trade and that there were clusters of foreign settlers. Perhaps they were traders, workers and/or mercenaries.

Strabo gave indications of the social, economic and ritual importance of horses and horse breeding among the pre-Roman Veneti. He wrote that they bestowed attention on horse rearing, "which, though now entirely abandoned, was formerly in great esteem among them, resulting from the ancient rage for breeding mules, which Homer thus mentions: "From the [Ve]neti for forest mules renowned." It was here that Dionysius, the tyrant of Sicily, kept his stud of race-horses. And, in consequence, the [Veneti] horses were much esteemed in Greece, and their breed in great repute for a long period." (Homer actually wrote "whence is the race of wild mules." Iliad II. 857) Strabo also noted that the Veneti paid honour to Diomedes by sacrificing a white horse.

In Altinum there was a large number of horses buried in a sacrificial pit at the town's sanctuary and in the cemetery area to the north of the town. In the latter there were some thirty horses, whereas in other Veneti towns there were only a few, except for a cemetery exclusively devoted to some thirty horses to the south of Este. These burials are dated to the 4th and 3rd centuries BCE. Evidence of sacrifice at the sanctuary continues into the Roman imperial period. Here the remains of some twenty horses were found alongside those of bovines, sheep, goats and pigs. The ritual offerings were the heads or parts of hindlegs or tails which were disarticulated, skinned and fleshed. Sketches of headless horses or body parts confirm this animal's key role in religious cults.

Archaeological finds indicate that Altinum's perimeter was marked by waterways beyond which there were cemetery areas. The abundance of watercourses gives the picture of a town deeply tied to water, which was characteristic of towns in the Veneto. The transformation of Altinum from a Veneti to a Roman town started with the reorganisation of its marshy environment through the regulation of its waters and the expansion of its canal network. The main work was the digging of the Silocello canal to link the River Sile (which was to the north of the town) to the channel, which today is called Santa Maria and which flowed to the south of the town from the River Dese to the lagoon. The most important architectural finds are a town gate on the canal which marked the northern edge of the town, another public building which faced the canal which marked the southern edge of the town, and a temple close to the Santa Maria channel which was restructured in a monumental form from a 5th-century BCE wooden structure. (See the "Port and canals" section for Altinum's canals)