In Egyptian mythology, Nephthys (spelt Nebet-het, and Nebt-het, in transliteration from hieroglyphs) is one of the Ennead of Heliopolis, a daughter of Nut and Geb, and the wife of Set. She was originally Set's dualistic counterpart, representing the air, whereas Set originally represented the desert. In ancient Egypt, the oldest female in the house was given the honorary title of Nephthys, and she was popular even in the Greco-Roman period.

Consequently she was named Nebt-het, which means lady of the house, the house being a colloquial term for the sky, a use also present in the name of Hathor, meaning house of horus. In art, she was depicted as a hawk, representative of the air, or as a woman with a hawk's wings, usually outstretched as a symbol of protection. She was shown crowned by the hieroglyphic of her name, which was the sign for a house (het), with the sign for neb, which could also mean basket, on top of it.

Because Set represented the barren desert, Set was seen at first as infertile, and subsequently as homosexual, and so Nepthys was seen as childless. As she was also seen as a bird, she gradually became seen as a vulture, which the Egyptians believed never had children because they were thought to all be female (the Egyptians thought that vultures were spontaneously created from air). Although vultures were seen in a positive light, their feeding behaviour nevertheless led them to be associated also with decay and death, and so Nepthys too gradually became a goddess of death and decay.

This led her, in art, to be depicted as a mourning woman, and her hair was compared to the strips of cloth that shroud the bodies of the dead. She was known as a Friend of the Dead, and professional mourners became referred to as the Hawks of Nephthys. When the Ennead and Ogdoad were merged, Nephthys was seen as joining the night-time boat journey of Ra, the sun god, when he entered the underworld, and accompanying him until he met the day again.

In the Ennead, she is also the sibling of the other pair - Osiris and Isis, who represented death and life, respectively. Consequently, when the merger of Ogdoad and Ennead caused the Ogdoad's god of death, Anubis, to be displaced, and become a lesser god, it was said that Nepthys was Anubis' mother, and Osiris his father. This was described in myth by stating that a sexually frustrated Nepthys disguised herself as Isis to appeal to Set, but he did not notice her, but Osiris, Isis' husband, did, mistaking her for Isis, and resulting in the birth of Anubis. Later alternative versions hold that she just drugged Osiris with wine in order to seduce him.

Later, Nepthys, as the air, was identified as the source of rain, which occurred frequently only south of Egypt, and so was seen as the creator of the Nile river. Consequently she became identified as the goddess Anuket, who was considered to be the source of the Nile. As a funerary goddess, this led to her being considered a protector of Hapi, one of the Four sons of Horus, specifically the deification of the canopic jar containing the lungs, the organ that suffers most from drowning.