The Kassites (/ˈkæsaɪts/) were a people of the ancient Near East. They controlled Babylonia after the fall of the Old Babylonian Empire from c. 1531 BC until c. 1155 BC (short chronology).

The Kassites gained control of Babylonia after the Hittite sack of Babylon in 1531 BC, and established a dynasty generally assumed to have been based first in that city, after a hiatus. Later rule shifted to the new city of Dur-Kurigalzu. By the time of Babylon's fall, the Kassites had already been part of the region for a century and a half, acting sometimes with Babylon's interests and sometimes against. There are records of Kassite and Babylonian interactions, in the context of military employment, during the reigns of Babylonian kings Samsu-iluna (1686 to 1648 BC), Abī-ešuh, and Ammī-ditāna.

The origin and classification of the Kassite language, like the Sumerian language and Hurrian language, is uncertain, and, also like the two latter languages, has generated a wide array of speculation over the years, even to the point of linking it to Sanskrit. However, like these other languages, it is regarded as a language isolate and is not accepted to be Semitic or Indo-European. The Kassite religion is also poorly understood, though the names of some Kassite deities are known. The chief gods, titular gods of the kings, were Shuqamuna and Shumaliya, which are distinct from Sumerian, Semitic and Indo-European gods. As was typical in the region, there was some cross-pollination with other religions. After Babylon came within the Kassite sphere of control its city-god, Marduk, was absorbed into the Kassite pantheon.

Documentation of the Kassite period depends heavily on the scattered and disarticulated tablets from Nippur, where thousands of tablets and fragments have been excavated. They include administrative and legal texts, letters, seal inscriptions, private votive inscriptions, and even a literary text (usually identified as a fragment of a historical epic). Many of those tablets have not yet been published, including hundreds held in the Ottoman Museum in Istanbul.

About 100 Kassite tablets were found at Dur-Kurigalzu. A few inscribed building materials of Kurigalzu I were found at Kish. Several tablets dated to the reign of Agum III were found at the Dilmun site of Qal'at al-Bahrain. In total, about 12,000 Kassite period documents have been recovered, of which only around 10% have been published. There are also a number of building inscriptions, all but one written in Sumerian unlike the Akkadian typically used by the Kassites. A number of seals have also been found. Kudurrus, stone stele used to record land grants and related documents, provide another source for Kassite history. This practice continued for several centuries after the end of the Kassite Dynasty. Often situated on the surface, many were found early and made their way to museums around the world.

The ancient city of Nippur was a major focus for the Kassites. Early on, refurbishments were conducted of the various religious and administrative buildings, the first of these datable to Kurigalzu I. Major construction occurred under Kadashman-Enlil, Kudur-Enlil, and Shagarakti-Shuriash, with lesser levels of repair work under Adad-shuma-usur and Meli-Shipak. Other important centers during the Kassite period were Larsa, Sippar and Susa. The Kassites were very active at Ur. At the site of Isin, which had been abandoned after the time of Samsu-iluna, major rebuilding work occurred on the religious district including the temple of Gula. The work at Isin was initiated by Kurigalzu I and continued by Kadashman-Enlil I, and after a lapse, by Adad-shuma-usur and Meli-Shipak II. After the Kassite dynasty was overthrown in 1155 BC, the system of provincial administration continued and the country remained united under the succeeding rule, the Second Dynasty of Isin.

The origin of the Kassites is uncertain, though a number of theories have been advanced. Several suspected Kassite names are recorded in economic documents from the Ur III period (c. 2112–2004 BC) in southern Babylon, but their origin is ambiguous. It has been suggested they originated from the Zagros Mountains region. Kassites were first reported in Babylonia in the 18th century BC, especially around the area of Sippar. The 9th year name of king Samsu-iluna (c. 1749–1712 BC) of Babylon, the son of Hammurabi mentions them ie. ("Year in which Samsu-iluna the king (defeated) the totality of the strength of the army / the troops of the Kassites").

As the Babylonian empire weakened in the following years the Kassites became a part of the landscape, even at times supplying troops for Babylon. The Babylonians divided those into kingdom resident Kassites (referred to as such) and Kassites from peripheral areas termed Samḫarû and Bimatü. It is known that a contingent of Bimatü were stationed at Dūr-Abī-ešuḫ. The idol of the god Marduk had been carried off from Babylon, possibly by the Hittites, but the Kassite rulers regained possession, returned Marduk to Babylon, and made him the equal of the Kassite Shuqamuna. Babylon under Kassite rulers, who renamed the city Karanduniash, re-emerged as a political and military power in Mesopotamia.