Interrex, plural interreges, in ancient Rome, a provisional ruler specially appointed for a period during which the normal constituted authority was in abeyance (the interregnum). The title originated during the period of the Roman kings when an interrex was appointed (traditionally by the Senate) to carry on the government between the death of one king and the election of his successor. It was subsequently used in republican times for an officer appointed to hold the comitia that would elect new consuls when for some reason the retiring consuls had not done so.

In the regal period when the Senate decided to appoint interreges, the Senate divided itself into 10 decuriae, from each of which one senator was selected. All were patricians, usually former consuls. Each of these 10 senators acted as king for 5 days; and if, at the end of 50 days, no king had been elected, the rotation was renewed. It was their duty to nominate a king, whose appointment was then ratified or refused by the curiae and the patrician senators. Under the republic the same procedures were followed when a consul could not complete his term of office. When the first consuls were elected, Spurius Lucretius may have held the comitia as interrex, and from that time down to the Second Punic War (218–201 bc) such officers were from time to time appointed. Thenceforward there is no record of the office until 82 bc, when the Senate appointed an interrex to hold the comitia, which made Sulla dictator. In 55, 53, and 52 bcinterreges are again found, the last-mentioned being on the occasion when Pompey was elected sole consul.