Phoneutria nigriventer is a species of medically significant spider in the family Ctenidae, found in the Southern Cone of South America. Along with other members of the genus, they are often referred to as Brazilian wandering spiders.

Wandering spiders (Ctenidae) are a family of spiders that includes the Brazilian wandering spiders. These spiders have a distinctive longitudinal groove on the top-rear of their oval carapace similar to those of the Amaurobiidae. They are highly defensive and venomous nocturnal hunters. Wandering spiders are known to hunt large prey, for example hylid species Dendropsophus branneri. Despite their notoriety for being dangerous, only a few members of Phoneutria have venom known to be hazardous to humans, but the venoms of this family are poorly known, so all larger ctenids should be treated with caution.