Quintus Fabius Ambustus, likely a member of the patrician gens Fabia, held the Roman office of dictator. The precise date and circumstances of his dictatorship are not definitively known, and the details surrounding his life are scant and subject to scholarly debate.
It is generally believed he held the office during the early to mid-4th century BC, a period marked by internal social tensions between patricians and plebeians, as well as external conflicts with various Italic tribes. Sources attribute the appointment of Fabius Ambustus as dictator to either a military crisis or, perhaps more plausibly, to oversee the comitia, the Roman assemblies, for the purpose of conducting elections or managing specific legislation. The limited information suggests his dictatorship may have been relatively short-lived, as was often the case with those appointed primarily for administrative purposes.
The lack of extensive historical records from this era makes it difficult to ascertain the precise reasons for his appointment and the specific actions he undertook while in power. He is typically distinguished from other members of the Fabii with the same or similar names by the designation "(dictator)". Further complicating matters is the possibility of confusion with other prominent members of the Fabii family who served as consuls or held other significant magistracies during the same period. Modern historians rely on fragmented sources and circumstantial evidence to reconstruct the events surrounding his dictatorship.