Mirian III is a king of Iberia or Kartli (Georgia), contemporaneous to the Roman emperor Constantine the Great (r. 306–337).

He is the founder of the royal Chosroid dynasty, also known as the Iberian Mihranids or Mihranids of Iberia.

According to the early medieval Georgian annals and hagiography, Mirian was the first Christian king of Iberia, converted through the ministry of Nino, a Cappadocian female missionary.

He is credited with establishment of Christianity as his kingdom's state religion and is regarded by the Georgian Orthodox Church as saint and is canonized as Saint Equal to the Apostles King Mirian.

Traditional chronology after Prince Vakhushti assigns to Mirian's reign—taken to have lasted for seventy-seven years—the dates 268–345, which Professor Cyril Toumanoff corrects to 284–361. He is also known to the contemporary Roman historian Ammianus Marcellinus and the medieval Armenian chronicles.