Latrodectus is a genus of spider, in the family Theridiidae, which contains 32 recognized species. The common name widow spiders is sometimes applied to members of the genus due to a behaviour seen in some species in which the female eats the male after mating. The black widow spider is perhaps the best-known member of the genus. Its bite is dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named for the genus. The female black widow has unusually large venom glands and its bite is particularly harmful to humans; however, Latrodectus bites rarely kill humans if proper medical treatment is provided.

The prevalence of sexual cannibalism in some species of Latrodectus has inspired the common name "black widow spider". The female's venom is at least three times more potent than that of the males, making a male's self-defense bite ineffective. Research at the University of Hamburg in Germany suggests this ultimate sacrifice strategy has evolved to promote the survival odds of the offspring; however, contrary to popular belief, females of some species rarely eat their partners after mating and much of the documented evidence for mate cannibalism has been observed in laboratory cages where the males could not escape.