The Ka'ba-ye Zartosht, meaning the "Cube of Zoroaster", is a 5th century B.C.E. Achaemenid-era tower-like construction. This enigmatic structure is one of many surviving examples of the Achaemenid architectural design.

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 square tower is constructed of white limestone blocks, that are held in place by iron cramps. Mortar was not used in its construction. Each side of the building is 7.25 meters wide. The 12.5 meter high structure has a slightly pyramidal roof and stands on a 1.5 meter high three-stepped plinth. Each face of the building is decorated with slightly recessed false windows of black limestone.

The structure has one square inner chamber, 5.70 meter high and 3.70 meter wide, access to which is through a doorway with a decorated lintel in the upper half of the tower. The chamber was once accessible by a flight of steps, only the lower half of which has survived. The 1.70 meter wide and 1.90 meter high door was of solid stone that was originally firmly closed but has since disappeared.

Iranian archaeologist Reza Moradi Ghiasabadi has presented his own theory and described the monument as the world's most unique calendrical and astronomical observatory. According to him the month, day and even certain times of the day can exactly be determined by the sunlight shed on the structure and its resulting shadows.

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