King of Kings was a ruling title employed primarily by monarchs based in the Middle East. Although most commonly associated with Iran (historically known as Persia in the West), especially the Achaemenid and Sasanian Empires, the title was originally introduced during the Middle Assyrian Empire by king Tukulti-Ninurta I (reigned 1233–1197 BC) and was subsequently used in a number of different kingdoms and empires, including the aforementioned Persia, various Hellenic kingdoms, Armenia, Georgia, and Ethiopia.
The title is commonly seen as equivalent to that of Emperor, both titles outranking that of king in prestige, stemming from the medieval Byzantine emperors who saw the Shahanshahs of the Sasanian Empire as their equals. The last reigning monarchs to use the title of Shahanshah, those of the Pahlavi dynasty in Iran (1925–1979), also equated the title with "Emperor". The rulers of the Ethiopian Empire used the title of Nəgusä Nägäst (literally "King of Kings"), which was officially translated as "Emperor". Sultan of Sultans is the sultanic equivalent of King of Kings.
In Judaism, Melech Malchei HaMelachim ("the King of Kings of Kings") came to be used as a name of God. "King of Kings" (βασιλεὺς τῶν βασιλευόντων) is also used in reference to Jesus Christ several times in the Bible, notably in the First Epistle to Timothy and twice in the Book of Revelation. In Islam, both the terms King of Kings and the Persian variant Shahanshah are condemned, particularly in Sunni hadith.
In Ancient India, Sanskrit language words such as Chakravarti, Rājādhirāja and Mahārādhirāja are among the terms that were used for employing the title of the King of Kings. These words also occur in Aitareya Aranyaka and other parts of Rigveda (1700 BC - 1100 BC).
The title King of Kings was first introduced by the Assyrian king Tukulti-Ninurta I (who reigned between 1233 and 1197 BC) as šar šarrāni. The title carried a literal meaning in that a šar was traditionally simply the ruler of a city-state.