The hammer-headed bat (Hypsignathus monstrosus), also known as hammer-headed fruit bat, big-lipped bat, and hammerhead bat, is a megabat widely distributed in West and Central Africa. It is the only member of the genus Hypsignathus, which is part of the tribe Epomophorini along with four other genera. It is the largest bat in continental Africa, with wingspans approaching 1 m (3.3 ft), and males are almost twice as heavy as females. Males and females also greatly differ in appearance, making it the most sexually dimorphic bat species in the world. These differences include several adaptations that help males produce and amplify vocalizations: the males' larynges (vocal cords) are about three times as large as those of females, and they have large resonating chambers on their faces. Females appear more like a typical megabat, with foxlike faces.

The hammer-headed bat is frugivorous, consuming a variety of fruits such as figs, bananas, and mangoes, though a few instances of carnivory have been noted. Females tend to travel a consistent route to find predictable fruits, whereas males travel more to find the highest quality fruit. It forages at night, sleeping during the day in tree roosts. Individuals may roost alone or in small groups. Unlike many other bat species that segregate based on sex, males and females will roost together during the day. It has two mating seasons each year during the dry seasons. It is believed to be the only bat species with a classical lek mating system, wherein males gather on a "lek", which in this case is a long and thin stretch of land, such as along a river. There, they produce loud, honking vocalizations to attract females. Females visit the lek and select a male to mate with; the most successful 6% of males are involved in 79% of matings. Offspring are born five or six months later, typically a singleton, though twins have been documented. Its predators are not well-known, but may include hawks. Adults are commonly affected by parasites such as flies and mites.

The hammer-headed bat is sometimes considered a pest due to its frugivorous diet and its extremely loud honking noises at night. In Nigeria and the Democratic Republic of the Congo, it is consumed as bushmeat. It has been investigated as a potential reservoir of the Ebola virus, with several testing positive for antibodies against the virus. It is not considered a species of conservation concern due to its large range and presumably large population size.

The hammer-headed bat was described as a new species in 1861 by American scientist Harrison Allen. Allen placed the species into a newly-created genus, Hypsignathus. The holotype had been collected by French-American zoologist Paul Du Chaillu in Gabon. The genus name Hypsignathus comes from the Ancient Greek ὕψος (húpsos, 'high'), and γνάθος (gnáthos, 'jaw'). T. S. Palmer speculated that Allen chose the name Hypsignathus to allude to the "deeply arched mouth" of the species. The species name monstrosus is Latin for "having the qualities of a monster".

A 2011 study found that Hypsignathus was the most basal member of the tribe Epomophorini, which also includes Epomops, Micropteropus, Epomophorus, and Nanonycteris. Initially, Allen identified the hammer-headed bat as a member of the subfamily Pteropodinae of the megabats. However, in 1997, Epomophorini was recognized as part of the subfamily Epomophorinae. Some taxonomists do not recognize Epomophorinae as a valid subfamily and include its taxa, including the Epomophorini, within Rousettinae.

The hammer-headed bat is the largest bat in mainland Africa. Males have wingspans up to 90.1 cm (2.96 ft), and all individuals have forearm lengths exceeding 112 mm (4.4 in). It has pronounced sexual dimorphism, more so than any other bat species in the world, with males up to twice as heavy as females. The average weight of males is 420 g (15 oz), compared to 234 g (8.3 oz) for females. Other differences between the sexes relate to their social system, in which males produce loud, honking vocalizations. Therefore, males have greatly enlarged larynges, about three times the size of females', extending through most of the thoracic cavity, and measuring half the length of the spine. The larynx is so large, it displaces other organs, including the heart, lungs, and gastrointestinal tract. Males also have resonating chambers to increase the volume of sound production. These chambers are pharyngeal air sacs connected to a large sinus in the humped snout. These numerous adaptations caused scientists Herbert Lang and James Chapin to remark, "In no other mammal is everything so entirely subordinated to the organs of voice".

Males overall have boxy heads with enormous lips, while the females, with their narrower snouts, have more foxlike faces. Males and females both have dark brown fur, with a paler mantle (sides and back of neck). It has patches of white fur at the base of the ears, though sometimes indistinct. The fur is long and smooth, though somewhat woolly in texture on the mantle. The ears are triangular and blackish-brown, and the eyes are very large. The dental formula is 2.1.2.12.1.3.2 for a total of 28 teeth; very occasionally, individuals have been found with an additional upper premolar on each side of the mouth, for a total of 30 teeth. The skull is larger and more robust than any other megabat in Africa, with a pronounced, massive snout. The tongue is large and powerful, with an expanded, tridentate tip. The tongue has backwards-facing papillae used to extract juice from fruits.

The wings are characterized by low aspect ratio, meaning that it has a smaller wingspan relative to the wing area. The wing loading is considered exceptionally high, meaning that it has a large body weight relative to the wing area. The wings are blackish brown in color. The thumb is approximately 128–137 mm (5.0–5.4 in) long. The wings attach to the hindlimbs at the second toe. It lacks a tail.