In ancient Rome a promagistrate (Latin: pro magistratu) was a person who was granted the power via prorogation to act in place of an ordinary magistrate in the field. This was normally pro consule or pro praetore, that is, in place of a consul or praetor, respectively. This was expedient developed, starting in 327 BC and becoming regular by 241 BC, that was meant to allow consuls and praetors to continue their activities in the field without disruption.

Prorogation created an official with no civilian authority or responsibility in Rome and allowed commanders to retain their position indefinitely, weakening the time-limited check that Romans had over their commanders. Prorogation's permission for a commander to remain with "expert knowledge of local conditions" also helped increase the chances of victory; in the late Republic, politics often motivated by the ambitions of individuals, decided whose commands were extended.

Sometimes men who held no elected public office – that is, private citizens (privati) – were given imperium and prorogued, as justified by perceived military emergencies. In the late republic, this was most exemplified by Pompey, who held a series of promagisterial commands before ever holding a magistracy or even joining the senate. With the acquisition of provinces outside of Italy and the expansion of the quaestiones perpetuae (permanent courts), it became normal for the provincial governors to be promagistrates. By the late republic, practically all governors were dispatched pro consule, regardless of their last urban magistracy.

The titles "proconsul" and "propraetor" are not used by Livy or literary sources of the republican era. Those Romans did not view a promagistracy a formal office in the republic but rather as an administrative expedient.