The Fasti Ostienses are a calendar of Roman magistrates and significant events from 49 BC to AD 175, found at Ostia, the principal seaport of Rome. Together with similar inscriptions, such as the Fasti Capitolini and Fasti Triumphales at Rome, the Fasti Ostienses form part of a chronology known as the Fasti Consulares, or Consular Fasti.

The Fasti Ostienses were originally engraved on marble slabs in a public place, either the Ostian forums, or the temple of Vulcan, the tutelary deity of Ostia. The fasti were later dismantled and used as building materials. Since their rediscovery, they have become one of the primary sources for the chronology of the early Roman Empire, along with historians such as Tacitus, Suetonius, and Cassius Dio.

The term fasti originally referred to calendars published by the pontifices, indicating the days on which business could be transacted (fasti) and those on which it was prohibited for religious reasons (nefasti). These calendars frequently included lists of the annual magistrates. In many ancient cultures, the most common way to refer to individual years was by the names of the presiding magistrates. The annually-elected consuls were the eponymous magistrates at Rome, and so lists of the consuls going back many years were useful for dating historical events. Over time such lists also became known as fasti.

Located at the mouth of the Tiber, Ostia was the chief seaport of Rome from the earliest period until the third century AD, when it was overtaken by Portus. The Fasti Ostienses were inscribed in a public place somewhere in the city, although precisely where is uncertain; perhaps in the local forum, or on the walls of the temple of Vulcan, the location of which has not been identified. In either case, they were probably superintended by the Pontifex Volkani, the priest of Vulcan at Ostia. The surviving fragments of the Ostienses mention this appointment several times. The carving of the Ostian fasti may have begun as early as the dictatorship of Sulla, in 81 BC, but the earliest surviving portion records the events from 49 to 44 BC. The last extant year is AD 175, but there are many gaps, and most of the surviving years are damaged.

It is not clear at what time the fasti were dismantled for reuse as building material; they may have been abandoned as early as the Severan dynasty, but more likely this occurred following the advent of Christianity as the state religion toward the end of the fourth century, or still later, when the city came under threat from raids from both land and sea during the fifth century. However, from the ninth century to the nineteenth, the old city was effectively abandoned, and regarded largely as a source of material for construction elsewhere.

For each year, the Ostienses provide a list of the consuls, including both of the ordinares, the consuls who entered office at the beginning of January, and traditionally gave their names to the year, followed by all of the suffecti, consuls who took office following the resignation or death of their predecessors in the course of the year. Under the Republic, consules suffecti were elected only if one of the ordinares died, or was forced to resign. But in imperial times, it became common for the emperors to appoint two, four, or even six pairs of consuls during the course of a year.

Part of the reason for increasing the number of consuls was to show favour to the Roman aristocracy, for whom holding the consulship for even a short period was a great honour; but the more practical reason was to fill the large number of important positions in the imperial bureaucracy that were traditionally held by ex-consuls.

Typically, each pair of consuls would enter office at the beginning, or Kalends, of a month, although sometimes consuls would take office on the Ides or Nones, or on rare occasions between these dates. Most of the emperors held the consulship several times, typically serving as one of the ordinares, and then resigning, often as early as the Ides of January.