Amurru, also known under the Sumerian name Martu (in Sumerian and Sumerograms: 𒀭𒈥𒌅), was a Mesopotamian god who served as the divine personification of the Amorites. In past scholarship it was often assumed that he originated as an Amorite deity, but today it is generally accepted that he developed as a divine stereotype of them in Mesopotamian religion. As such, he was associated with steppes and pastoralism, as evidenced by his epithets and iconography. While this was initially his only role, he gradually developed other functions, becoming known as a god of the mountains, a warlike weather deity and a divine exorcist.

He is first attested in documents from the Ur III period, chiefly in Sumerian and Akkadian theophoric names. Later he also came to be worshiped in Babylon, Assur and other locations in Assyria and Babylonia. He had his own cult center somewhere in the area known as the Sealand in Mesopotamian texts.

Only a single myth about Amurru is known. It describes the circumstances of his marriage to Adgarkidu, the daughter of Numushda, the city god of Kazallu. Other sources attest different traditions about the identity of his wife. The goddess Ashratum is particularly well attested in this role. His father was the sky god Anu and it is presumed that his mother was usually Urash.

Amurru was a divine representation of the Amorites, a group living in areas west of Mesopotamia. The names Amurru (Akkadian) or Martu (Sumerian) could refer both to the god and to the people. The origin of both these words is unknown, and according to Paul-Alain Beaulieu neither of them has a plausible Sumerian, Akkadian or West Semitic etymology. There is also no indication that either of them ever served as the endonym of the groups they described.

In older literature, as late as the 1980s, it was commonly assumed that Amurru originally an eponymous deity of the Amorites themselves. However, there is general agreement that he was instead a Mesopotamian god representing the westerners.

Amurru is absent from Amorite names from the Ur III period. The evidence from the Old Babylonian period is similarly lacking: while around seven thousand linguistically Amorite names are known, none of them are theophoric names invoking Amurru. However, he appears within many Sumerian theophoric texts as Martu, especially in texts from Lagash. He is also attested in Akkadian names, though even in this case his popularity appears to be smaller in areas where a higher percentage of population was Amorite, for example in the kingdom of Mari, while in Nippur, where very few, if any, Amorites lived, they are common. Tonia Sharlach notes that the perception of Amorites in Mesopotamia is a complex issue. While literary texts describe them as archetypal barbarians, and walls were built on the borders to prevent their entry, king Shulgi was supposedly proficient in the Amorite language, and people of Amorite origin held various offices in the royal administration, and could serve as priests. Most likely the creation of a god representing them was meant to provide them with a symbolic place in Mesopotamian religion due to their growing political importance.

Other analogous deities are also attested: Kaššû and Kaššītu, a pair of deities, respectively male and female, represented the Kassites, Aḫlamayītu was "the Aramean goddess," while Sutītu - "the Sutean goddess." However, these deities only emerged in the first millennium BCE, and are not attested earlier.

In texts from the Ur III and Old Babylonian periods, Amurru chiefly functioned as a divine stereotype of Amorites. However, he gradually acquired other functions, possibly due to the growing power of Amorite dynasties in the early second millennium BCE and due to assimilation of Amorite groups into Mesopotamian society. In the Kassite period, when Amorites ceased to function as a distinct group in Mesopotamia, Amurru lost his initial function as a representation of them. As early as in the Old Babylonian period, he came to be viewed as a divine exorcist. This became his primary role at least until the reign of Sennacherib.