Ardashir I ( اردشير یکم) (died 242 AD) was the founder of the Sassanid Empire, the ruler of Istakhr (since 206?), subsequently Fars Province (since 208?), and finally "King of Kings of Sassanid Empire" (after 226) with the overthrow of the Parthian Empire. The dynasty founded by Ardashir would rule for four centuries, until it was overthrown by the Rashidun Caliphate in 651.

Ardashir (Arđaxšēr from Middle Persian and Parthian Artaxšaθra, Pahlavi ʼrthštr, "Who has the Divine Order as his Kingdom") is also known as Ardeshīr-i Pāpagān "Ardashir, son of Pāpağ", and other variants of his name include Latinized Artaxares and Artaxerxes.