Interrex or "inter-rex" (Latin; plural, interreges) was literally a ruler "between kings." He was in effect a kind of regent.

The institution of interrex originated in the Kingdom of Rome. When a king died, the Roman Senate appointed an interrex to serve for an indefinite period (usually less than a year). The interrex could nominate whomever he wished to the Roman Assembly, which then elected a king. As soon as the new king entered upon his office, the position of interrex was vacated.

The institution of interrex, with some modifications, was carried over into the Roman Republic. If no consuls had been elected by New Year's Day, for whatever reason (e.g., civil strife, filibustering), a patrician senator was elected (or appointed) by the Senate to serve as interrex for five days. After that, another such interrex was elected, again for five days. The second interrex was charged with organizing new elections. If still no elections had been held, further new interreges were appointed every five days until they had been held.

A similar institution of interrex later existed in the Polish-Lithuanian Commonwealth, whose ruling classes liked to view their Republic (Rzeczpospolita) or Commonwealth as an heir to Roman republican traditions. The Commonwealth's kings entered upon their office by free election, which often led to a relatively long interregnum. Traditionally the role of interrex was carried out by the primate of the Roman Catholic Church in Poland.