Alba Longa (occasionally written Albalonga in Italian sources) was an ancient Latin city in Central Italy in the vicinity of Lake Albano in the Alban Hills. The ancient Romans believed it to be the founder and head of the Latin League, before it was destroyed by the Roman Kingdom around the middle of the 7th century BC and its inhabitants were forced to settle in Rome. In legend, Romulus and Remus, founders of Rome, had come from the royal dynasty of Alba Longa, which in Virgil's Aeneid had been the bloodline of Aeneas, a son of Venus.

According to Livy, Roman patrician families such as the Julii, Servilii, Quinctii, Geganii, Curiatii and Cloelii originated in Alba Longa.

Livy said of Alba Longa that it was founded by Ascanius to relieve crowding at Lavinium. He placed it at the foot of the Alban Mount and said that it took its name from being extended along a ridge. Dionysius of Halicarnassus repeated the story, but added that Ascanius, following an oracle given to his father, collected other Latin populations as well. Noting that alba means "white" and longa means "long", he translated the name into the Greek language as "long white town". Dionysius placed the town between the Alban Mount and the Alban Lake, thus beginning a long controversy about its location.

There is, however, "no archaeological evidence of a powerful state in the area of the Alban Hills at this time". It is unlikely, in the opinion of classicist Tim Cornell, that there was any urbanised city-state capable of holding hegemony in the region at the time. Nor is there any convincing evidence to connect the modern town of Albano Laziale with the ancient Alba Longa. Much of the prominence of Alba Longa in the ancient world was due to its status as a religious centre, as it hosted the annual Latin Festival on the mons Albanus in honour of Jupiter Latiaris.

Archaeological data show the existence of a string of villages in the Iron Age, each with its own necropolis, along the south-western shore of Lake Albano. At the time it was destroyed by Rome, these villages must have still been in a pre-urban phase, beginning to group around a centre which may well have been Castel Gandolfo, whose significantly larger necropolis suggests a larger town.

In the later republican period the territory of Alba (the Ager Albanus) was settled once again with many residential villas, which are mentioned in ancient literature and of which remains are extant.

Since the 16th century, the site has been at various times identified as that of the Convent of St. Paul at Palazzola near Albano, Coste Caselle near Marino, and Castel Gandolfo. The last named of these places in fact occupies the site of the Villa of Domitian which, according to Juvenal, was situated on the arx of Alba.

The Roman tradition held that Alba Longa was an urbanised city-state which founded a number of Latin colonies which it then organised into a Latin League. This narrative is implausible, likely anachronistic, and "cannot be historically true in a literal sense". Archaeological evidence indicates the presence of small villages in the area of the Alban hills during the late Bronze and early Iron Ages, but they failed to develop into cities and "are certainly unlikely to have founded Rome".