A quaestor (British English: /ˈkwiːstər/ KWEE-stər, American English: /ˈkwistər/; Latin: [ˈkʷae̯stɔr]; "investigator") was a public official in ancient Rome. There were various types of quaestors, with the title used to describe greatly different offices at different times.

In the Roman Republic, quaestors were elected officials who supervised the state treasury and conducted audits. When assigned to provincial governors, the duties were mainly administrative and logistical, but also could expand to encompass military leadership and command. It was the lowest ranking position in the cursus honorum (course of offices); by the first century BC, one had to have been quaestor to be eligible for any other posts.

In the Roman Empire, the position initially remained as assistants to the magistrates with financial duties in the provinces, but over time, it faded away in the face of the expanding imperial bureaucracy. A position with a similar name (the quaestor sacri palatii) emerged during the Constantinian period with judicial responsibilities.

Quaestor derives from the Latin verb quaero, quaerere, meaning "to inquire" (probably ultimately from the Proto-Indo-European root of interrogative pronouns *kʷo-). The job title has traditionally been understood as deriving from the original investigative function of the quaestores parricidii.

The earliest quaestors were quaestores parricidii, chosen to investigate capital crimes, and may have been appointed as needed rather than holding a permanent position. Under the Republic, these quaestores parricidii persisted, as prosecutors for capital cases in trials before the people. They disappear, however, by the second century BC.

Ancient authors disagree on the exact manner of selection for this office as well as on its chronology, with some dating it to the mythical reign of Romulus. This view, however, is "not at all credible" and there is no clear evidence for a specific date for the quaestorship's beginning.

The classical quaestors with financial responsibilities may be unconnected with the older questores parricidii. However, the debate still continues, but has more recently trended against connecting the two offices, which are connected by nothing other than a name. The two general theses are that the classical quaestorship related with financial matters either was created entirely separately from the older judicial quaestorship or that it evolved from that older quaestorship to meet greater administrative needs.

The traditional cursus honorum (career path) was loosely regulated, but after 197 BC, became more so, with a basic progression that one first had to hold the quaestorship before being considered for higher office as praetor or consul, with quaestor as the lowest office. After Sulla's reforms, the cursus honorum was cemented, with the added requirement that to stand for the quaestorship, one first needed to have been one of the vigintiviri and have held the military tribunate. The reforms also established that the minimum age for candidates had to be 30.