Mictlantecuhtli ("lord of Mictlan"), in Aztec mythology, was a god of the dead and King of Mictlan (Chicunauhmictlan), the lowest and northernmost section of the underworld. He was one of the principal gods of the Aztecs and was the most prominent of several gods and goddesses of death and the underworld (see also Chalmecatl). The worship of Mictlantecuhtli sometimes involved ritual cannibalism, with human flesh being consumed in and around the temple.

He was depicted as a blood-spattered skeleton or a person wearing a toothy skull. His headdress was shown decorated with owl feathers and paper banners, and he wore a necklace of human eyeballs. He was not the only Aztec god to be depicted in this fashion, as numerous other deities had skulls for heads or else wore clothings or decorations that incorporated bones and skulls. Although such imagery might seem morbid today, in the Aztec world skeletal imagery was a symbol of fertility, health and abundance, alluding to the close symbolic links between death and life.

His wife was Mictecacihuatl, and together they were said to dwell in a windowless house in Mictlan. Mictlanteculhtli was associated with spiders, owls, bats, the eleventh hour, and the northern compass direction. He was one of only a few deities held to govern over all three types of souls identified by the Aztecs, who distinguished between the souls of people who died normal deaths (of old age, disease, etc), heroic deaths (e.g. in battle, sacrifice or during childbirth), or non-heroic deaths.

Mictlanteculhtli was the god of the day sign Itzcuintli (dog), one of the 20 such signs recognised in the Aztec calendar, and was regarded as supplying the souls of those who were born on that day. He was seen as the source of souls for those born on the sixth day of the 13-day week and was the fifth of the nine Night Gods of the Aztecs. He was also the secondary Week God for the tenth week of the twenty-week cycle of the calendar, joining the sun god Tonatiuh to symbolise the dichotomy of light and darkness.

According to Aztec legend, the twin gods Quetzalcoatl and Xolotl were sent by the other gods to steal the bones of the previous generation of gods from Mictlanteculhtli. The god of the underworld sought to block Quetzalcoatl's escape with the bone and, although he failed, he forced Quetzalcoatl to drop the bones, which were scattered and broken by the fall. The shattered bones were collected by Quetzalcoatl and carried back to the land of the living, where the gods transformed them into the various races of mortals.