The 1383–1385 Portuguese interregnum was a civil war in Portuguese history during which no crowned king of Portugal reigned. The interregnum began when King Ferdinand I died without a male heir and ended when King John I was crowned in 1385 after his victory during the Battle of Aljubarrota.

An interregnum is a period of discontinuity or "gap" in a government, organization, or social order. Archetypally, it was the period of time between the reign of one monarch and the next, and the concepts of interregnum and regency therefore overlap. Historically, longer and heavier interregna have been typically accompanied by widespread unrest, civil and succession wars between warlords, and power vacuums filled by foreign invasions or the emergence of a new power. A failed state is usually in interregnum.