Ninsun (also called Ninsumun, cuneiform: 𒀭𒊩𒌆𒄢 ^dNIN.SUMUN_2; Sumerian: Nin-sumun(ak) "lady of the wild cows") was a Mesopotamian goddess. She is best known as the mother of the hero Gilgamesh and wife of deified legendary king Lugalbanda, and appears in this role in most versions of the Epic of Gilgamesh. She was associated with Uruk, where she lives in this composition, but she was also worshiped in other cities of ancient Mesopotamia, such as Nippur and Ur, and her main cult center was the settlement KI.KAL^ki.

The degree of Ninsun's involvement in Gilgamesh's life varies between various versions of the Epic. She only plays an active role in the so-called "Standard Babylonian" version, in which she advises her son and interprets his dreams, petitions the sun god Shamash to protect him, and accepts Enkidu as a member of her family. In the Old Babylonian version her role is passive, with her actions being merely briefly discussed by Shamhat, while a Hittite translation of the text omits her altogether. She is additionally present in older Sumerian compositions, including Gilgamesh and the Bull of Heaven, as well as a poorly preserved and very early myth describing her first meeting with Lugalbanda and their marriage.

Kings from the Third Dynasty of Ur regarded Ninsun as their divine mother, and Gilgamesh as their brother, most likely to legitimize their claim to rule over Mesopotamia. Ur-Nammu and Shulgi both left behind inscriptions attesting their personal devotion to this goddess, and a prince only known from a single attestation bore the theophoric name Puzur-Ninsun.

The god list An = Anum mentions multiple children of Ninsun and her husband Lugalbanda separately from Gilgamesh. A sparsely attested tradition additionally regarded her as the mother of the dying god Dumuzi, indicating a degree of conflation with his usual mother Duttur. She could also be equated with the medicine goddess Gula, especially in syncretic hymns.

Ninsun's name was written ^dNIN.GUL. The cuneiform sign GUL could be read as both sun and sumun in Sumerian, as attested by syllabic glosses in lexical lists, but its meaning was the same in both cases, "wild cow". It is possible the name was initially understood as "lady wild cow", but in most of the ancient sources it was interpreted as a genitive compound, "lady of wild cows". Ninsun was often compared to these animals, and in the Epic of Gilgamesh she is at one point referred to with the Akkadian epithet Rimāt-Ninsun, "wild cow Ninsun".

In texts from Lagash, Ninsun is sometimes referred to as a lamma. In this context, lamma most likely should be understood as a designation of a deity's function, namely their involvement in granting long and prosperous life to devotees. It is possible that "Lamma-Ninsumuna" was depicted leading Lugalbanda by the wrist, even though lamma goddesses were usually described as walking behind the person they protected. It is also probable that in some cases Ninsun was believed to bestow a lamma upon kings. An inscription of Ur-Ningirsu I identifies her with the goddess Lammašaga, usually viewed as the sukkal of Bau. Claus Wilcke argues that in this case the name Lammašaga should be only understood as a descriptive epithet.

The so-called "Pennsylvania tablet" of the Old Babylonian version of the Epic of Gilgamesh attests that Ninsun was believed to be capable of dream interpretation.

Kings of the Third Dynasty of Ur, as well as Gudea of Lagash, regarded Ninsun as their divine mother. However, there is no evidence that Ninsun was ever regarded as a mother goddess similar to Aruru or Ninhursag.