Latrodectus is a broadly distributed genus of spiders with several species that together are referred to as true widows. This group is composed of those often loosely called black widow spider, brown widow spiders, and similar spiders. However, such general “common names” are of limited use as the diversity of species is much greater. A member of the family Theridiidae, this genus contains 32 species, which include several North American “black widows” (southern black widow Latrodectus mactans, western black widow Latrodectus hesperus, and northern black widow Latrodectus variolus). In addition to these in North America are also the Red Widow Latrodectus bishopi and the Brown Widow Latrodectus geometricus, which, in addition to North America, has a much wider geographic distribution. Elsewhere, others include the European black widow (Latrodectus tredecimguttatus), the Australian redback black widow (Latrodectus hasseltii), several different species in Southern Africa that can be called Button spiders, and the South American black widow spiders (Latrodectus corallinus and Latrodectus curacaviensis). Species vary widely in size. In most cases, the females are dark-coloured, but some may have lighter bodies or even reddish. Many can have red, white or brown markings on the upper-side (dorsal) of the abdomen. Some can be readily identifiable by reddish markings on the central underside (ventral) abdomen, which are often hourglass-shaped.

These small spiders have an unusually potent venom containing the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus. Female widow spiders have unusually large venom glands and their bite can be particularly harmful to large vertebrates, including humans. Only the bites of the females are dangerous to humans. Despite their notoriety, Latrodectus bites rarely cause death or produce serious complications.