The Sassanid Empire (also spelled Sasanid Empire, Sassanian Empire, or Sasanian Empire), known to its inhabitants as Ērānshahr and Ērān in Middle Persian and resulting in the New Persian terms Iranshahr and Iran, was the last pre-Islamic Persian Empire, ruled by the Sasanian Dynasty from 224 CE to 651 CE. The Sassanid Empire, which succeeded the Parthian Empire, was recognized as one of the two main powers in Western Asia and Europe, alongside the Roman Empire and its successor, the Byzantine Empire, for a period of more than 400 years.

The Sassanid Empire was founded by Ardashir I, after the fall of the Arsacid Empire and the defeat of the last Arsacid king, Artabanus IV. It lasted until Yazdegerd III lost control of his empire in a series of invasions from the Arab Caliphate. During its existence, the Sassanid Empire encompassed all of today's Iran, Afghanistan, Iraq, Syria, the Caucasus (Armenia, Georgia, Azerbaijan, and Dagestan), southwestern Central Asia, part of Turkey, certain coastal parts of the Arabian Peninsula, the Persian Gulf area, and areas of southwestern Pakistan. The native name for the Sassanid Empire in Middle Persian is Eran Shahr which means Aryan Empire. According to legend, the vexilloid of the Sassanid Empire was the Derafsh Kaviani. It was also hypothesized that the transition toward the Sassanid Empire represents the end of struggle of ethnic proto-Persians with their close migrant ethnic relatives, the Parthians, whose original homeland was in modern-day Central Asia.