Bellagio (Italian: [belˈlaːdʒo]; Comasco: Belàs [beˈlaːs]) is a comune (municipality) in the Province of Como in the Lombardy region of Italy. It is situated on Lake Como, known also by its Latin name Lario, where the lake's two southern arms branch, creating the Triangolo lariano. Bellagio's location at the tip of this promontory, looking out across the lake's northern arm towards the Alps, has long been noted for its scenic beauty. Bellagio is part of the Comunità montana del Triangolo lariano, headquartered in Canzo.

Traces of human presence around Bellagio date back to the Paleolithic period, approximately 30,000 years ago. Around the 7th to 5th centuries BC, a castellum (fortified hilltop settlement) may have existed on the promontory, potentially serving as a place of worship and trade for the scattered villages around the lake.

From around 400 BC, the Insubres, a Celtic tribe, are identified as the first inhabitants of the Bellagio area. They occupied the western shore of Lake Como up to its central point. The Orobii inhabited the northern arm of the lake and its eastern bank. The Insubres maintained their independence until the arrival of the Gauls led by Belloveso around 600 BC. The Gauls subjugated the Insubres, establishing themselves in Milan and Como, and garrisoning Bellagio as the northernmost point of their territory. This Gallo-Insubrian culture blended with the existing population, leaving traces in local place names such as Crux Galli (now Grosgalla) near Lezzeno, and Gallo, a small chapel on the old road to Limonta marking the border between municipalities.

In 225 BC, the Romans began their expansion northward, leading to the occupation of Gallo-Insubrian territory. Roman forces under Consul Marcus Claudius Marcellus defeated the Gallo-Insubres near Camerlata, taking control of Como and Lake Como's shores. During the Second Punic War, the Insubres briefly allied with Hannibal in hopes of regaining independence, but their aspirations were quashed by defeat in 194 BC, and the territory was fully incorporated into a Roman province by 80 BC.

Bellagio became a Roman garrison and a transit point for Roman armies heading to Raetia and the Splügen Pass. Troops overwintered near present-day Villa Serbelloni, benefiting from shelter from northerly winds and the area's mild climate. The Latin names Belacius and Bislacus suggest a possible origin in "Bi-lacus" ("between the lakes").

Between 81 and 77 BC, Cornelius Scipio established a colony of 3,000 Latin settlers around Lake Como. Julius Caesar, as proconsul from 59 BC, further augmented the population with 5,000 colonists, including 500 Greeks from Sicily. These diverse origins contributed to Bellagio's evolving ethnic makeup. Its strategic importance grew, serving not only as a wintering location but also as a harbor for warships, particularly at Loppia, where a natural inlet facilitated repairs. Loppia developed into one of Bellagio's earliest suburbs.

The Romans introduced Mediterranean crops such as olives and bay laurel. The Latin name for bay laurel, Laurus, is believed to be the origin of Lake Como's Latin name, Larius. Other introductions included the chestnut, cypress, and various herbaceous plants.

During the early Roman Empire, Virgil and Pliny the Younger brought renown to Lake Como and Bellagio. Virgil mentioned "great Lario" in his Georgics. Pliny the Younger maintained a summer villa, known as "Tragedy", near Bellagio's summit, where he spent extended periods studying, writing, hunting, and fishing, as documented in his letters regarding his Bellagio villas.