Annunitum (𒀭𒉡𒉌𒌈; also romanized as Anunītu) was a Mesopotamian goddess associated with warfare. She was initially an epithet of Ishtar of Akkad exemplifying her warlike aspect, but by the late third millennium BCE she came to function as a distinct deity. She was the tutelary goddess of the cities of Akkad and Sippar-Amnanum, though she was also worshiped elsewhere in Mesopotamia.

As attested in cuneiform texts from the Old Akkadian period onward, Annunitum's name was typically written as an-nu-ni-tum. Starting with the Old Babylonian period it was prefaced with the "divine determinative" (dingir). While an-nu-ni-tum remained the most common spelling in the Kassite period as well, in sources from the first millennium BCE a-nu-ni-tum, already known from a single Old Babylonian text and from a late Kassite inscription of king Meli-Shipak, became standard. An Old Babylonian prayer (CBS 19842) additionally preserves the shortened form a-nu-na, romanized by Wilfred G. Lambert as Anūna. There is no certainty whether the original pronunciation is better reflected by romanization with a single or double n.

It is presumed that Annunitum's name is derived from the root ‘nn, "warlike". It can be translated as "skirmisher" or "the martial one". Etymological connections with phonetically similar theonyms Anu, Antu and Anunna have been ruled out.

Annunitum was originally an epithet of Ishtar as the tutelary goddess of the city of Akkad. However, later it came to be viewed as a theonym, rather than an epithet, and developed into a distinct deity. It is assumed the process of separation of Annunitum from Ishtar started during the reign of Shar-Kali-Sharri (c. 2175-2150 BCE), though it is possible a passage from the reign of Naram-Sin (c. 2284–2275 BCE) already refers to them as a pair of distinct goddesses as opposed to a singular Ishtar-Annunitum. By the end of the third millennium BCE, Annunitum was widely recognized as a separate goddess. In the Ur III period, she was worshiped separately from Ishtar in Nippur, Ur and Uruk, and by the Old Babylonian period both of them had separate temples in Sippar, respectively the E-ulmaš and E-edina.

Spencer J. Allen refers to the phenomenon of epithets of Mesopotamian deities becoming distinct figures as "divine splintering". Other comparable cases of an epithet of Ishtar becoming a distinct deity include Dīrītu, Urkayītu and Arbilītu. Tonia Sharlach in her study of Annunitum and other goddesses worshiped in the court of Shulgi notes that it might be more accurate to refer to the names Ishtar and Inanna as "something of an umbrella term" designating multiple interconnected deities. For example, a collection of hymns from the Ur III period treats Inanna of Uruk, Inanna of Zabalam and Inanna of Ulmaš (Ishtar of Akkad) as three separate deities, with separate compositions dedicated to each of them. Sharlach suggests that to accommodate this information, the study of "forms" of Ishtar in Mesopotamian sources requires relying on a methodology developed by Gary Beckman for the purpose of study of deities designated by the logogram ^dIŠTAR in Hittite texts, such as the Hurrian Šauška. As summarized by Beckman, "in some respects (...) Ištar-figures partake of a common essence, while in others they are distinct". He also notes that "any special features of the varieties will become apparent only if each is initially studied in isolation".

Annunitum was regarded as a warlike goddess. She could be addressed as the lady of battle. She originally exemplified the martial side of Ishtar. An inscription of Nabonidus describes her as armed with a bow and arrows. An earlier cylinder seal from Sippar assumed to depict her in the company of another goddess, likely Aya, shows her holding a trident-like weapon. Another possible depiction of Annunitum on a cylinder seal shows her standing on the back of two addorsed lion-griffons. Other seals from Sippar indicate she could be depicted in a robe leaving one shoulder and breast exposed, similarly to Ishtar and Aya. It is assumed that this garment was meant to highlight beauty, charm and sex appeal.

In Mesopotamian astronomy Annunitum's name was used to refer to a constellation corresponding to part of Pisces, specifically the eastern fish. An ikribu prayer dedicated to her in this role is known. However, she was usually not described as an astral deity, in contrast with Ishtar.

A curse formula on a kudurru from the Kassite period indicates that Annunitum was among the deities regarded as capable of inflicting oath-breakers with leprosy, a role shared with deities such as Sin, Shamash, Ishtar, Anu and Enlil.