A bolas spider is a member of the orb-weaver spider (family Araneidae) that, instead of spinning a typical orb web, hunts by using one or more sticky "capture blobs" on the end of a silk line, known as a "bolas". By swinging the bolas at flying male moths or moth flies nearby, the spider may snag its prey rather like a fisherman snagging a fish on a hook. Because of this, they are also called angling or fishing spiders (although the remotely related genus Dolomedes is also called a fishing spider). The prey is lured to the spider by the production of up to three sex pheromone-analogues.

Bolas spiders have been treated as either the whole or part of either the tribe "Mastophoreae" or Mastophorini, the subfamily Mastophorinae, or the informal group mastophorines. Recent studies show that the genus Celaenia, which does not use a bolas, belongs in the same taxonomic group.

Bolas spiders are small nocturnal animals with conspicuous outgrowths on the upper (dorsal) surface of the cephalothorax. Mature females have a body length of 10–20 mm (1⁄2–3⁄4 in), while the males, which are much smaller, have a body length usually less than 2 mm (1⁄16 in). The abdomen of adult females is large relative to the cephalothorax, partially covering it. In some species, the female's abdomen is comparatively smooth, while in others it has humps and tubercules. The females of some bolas spiders (e.g. Mastophora cornigera) look remarkably like a bird dropping, thanks to their large, globular abdomen and brownish cephalothorax. This is a form of defensive mimicry as the animals that prey on spiders pay little attention to bird droppings, which enables the spiders to rest unnoticed during the day in fairly exposed places. M. bisaccata resembles the shell of a snail that is abundant in the spider's habitat. While the bird dropping mimics rest on the upper surface of leaves, M. bisaccata rests on the undersides. If removed from the surface, Mastophora females produce a pungent odor, which is highly unusual for spiders (other than bolas spiders, only one species of Cyrtarachne is known to do this). They do not attempt to flee when handled. Some species of Cladomelea also rest exposed on leaves during the day. However, the Australian Ordgarius magnificus, which displays eye spots on its back that make it resemble the moth it hunts, ties leaves together with silk to form a retreat.

Bolas-using spiders are species of four related genera. (Distributions are from the World Spider Catalog.)

The use of a bolas by the Madagascan genus Exechocentrus was not reported until 2012, so the genus is not included in earlier accounts of bolas spiders.

Bolas spiders are members of the family Araneidae, the orb-weaver spiders, although they do not weave webs. They may be treated as a group within the subfamily Cyrtarachninae sensu lato, or as the whole or part of a separate taxon. In 1931, Mello-Leitão created a tribe under the name Mastophoreae, which included genera now known to use a bolas as well the genus Acantharachne. Gertsch in 1955 also placed bolas spiders in the tribe Mastophoreae, again including Acantharachne (under the synonym Acantharanea). Article 29.2 of the International Code of Zoological Nomenclature states that the ending "-ini" should be used for tribes, so the correct name for the tribe is now "Mastophorini". The tribe has been elevated to a separate subfamily Mastophorinae. The informal name "mastophorines" has also been used, including both bolas-using spiders and a related genus.

A 2014 molecular phylogenetic study of a representative set of genera placed in Cyrtarachninae s.l. showed that the species of the two bolas spider genera included in the study formed a strongly supported clade nested within Cyrtarachninae s.l. A wider study of the family Araneidae published in 2020 reached a similar conclusion, separating Cyrtarachninae s.l. into two informal sister groups "mastophorines" and "cyrtarachines". Mastophorines included the genus Celaenia that does not use a bolas. Bolas-using spiders thus form a subgroup within the narrowly defined mastophorines or the broadly defined Cyrtarachninae.

The Cyrtarachninae, defined broadly, construct several kinds of web, which differ from the orb webs of other Araneidae. Some construct "spanning-thread webs", which have a small number of radii and widely spaced sticky threads that do not form spirals. Others construct triangular webs. These are formed in the same way as spanning-thread webs, but with only three radii, so that they appear triangular. Bolas spiders do not spin webs at all, adult females catching their prey on single sticky threads. Finally, some species capture their prey without a web, using their outstretched legs, as do juvenile and male bolas spiders.