The Ka'ba-ye Zartosht (alt: Kaba-ye Zardusht, Kaba-ye Zardosht), meaning the "Cube of Zoroaster", is a 5th century BCE Achaemenid-era tower-like construction at Naqsh-e Rustam, an archaeological site just northwest of Persepolis, Iran.
The name Ka'ba-ye Zartosht probably dates to the 14th century, when many pre-Islamic sites were identified with figures and events of the Qur'ān or the Shāhnāme. The structure is not actually a Zoroastrian shrine, nor are there reports of it ever having been a pilgrimage site.
The structure, which is a copy of a sister building at Pasargadae, was built either by Darius I (r. 521-486 BCE) when he moved to Persepolis, by Artaxerxes II (r. 404–358 BCE) or Artaxerxes III (r. 358–338 BCE). The building at Pasargadae is a few decades older. The wall surrounding the tower dates to Sassanid times.
The structure has a one square inner chamber, 5.70 m high and 3.70 m wide, access to which is through a doorway with a decorated lintel in the upper half of the tower. The chamber once accessible by a flight of steps, only the lower half of which has survived. The 1.70 m wide and 1.90 m high door was of solid stone that was originally firmly closed but has since disappeared.
From a reference to fire altars in a Sassanid-era inscription on the building it was inferred that the structure was once a fire altar, or perhaps as an eternal-flame memorial to the emperors whose tombs are located a few meters away. This theory has however since been rejected since the lack of cross-ventilation would have soon choked the flame, and in any case, the author of the inscription is unlikely to have known the purpose of the building seven centuries after its construction.
A later opinion suggested that both it and its sister building were safety boxes for the "paraphernalia of rule".
Today, "most scholars" consider the structure to be an Achaemenid royal tomb, and it has been observed that both the Ka'ba-ye Zartosht and its sister building at Pasargadae "more closely correspond to the description of Cyrus the Great's tomb by Arrian (6.29) and Strabo (15.3.7) than does the monument in Pasargadae which is commonly attributed to this king."
The trilingual inscription ('KZ') of Shapur I (241-272) is on the eastern (Middle Persian text), western (Parthian text) and southern (Greek text) walls. A Middle Persian inscription of the high priest Kartir - the 'KKZ' inscription - is below Shapur's on the eastern wall.