Bombus vestalis, the vestal cuckoo bumblebee, is a species of cuckoo bumblebee that lives in most of Europe, as well as North Africa and western Asia. It is a brood parasite that takes over the nests of other bee species. Its primary host is Bombus terrestris (the buff-tailed bumblebee). After its initial classification as Psithyrus vestalis, this bumblebee was recently^[when?] reclassified into the genus Bombus, subgenus Psithyrus.

The queen is a large bumblebee with a length up to 21 mm (0.83 in) and a wingspan of 37 mm (1.5 in); the male is considerably smaller (16 mm (0.63 in)). The bumblebee is predominantly black, with an orange collar. The third tergite has a border of yellow hairs, and the hairs on the fifth tergite are mostly white. The males are similar to the females, but smaller and with longer antennae.

It is similar in appearance to another cuckoo bumblebee, Bombus bohemicus, but is distinguishable by either the length of the antennal segments or dissection and comparison of the genitalia. In Bombus vestalis, the fifth antennal segment will be the same length as the third and fourth together.

A key characteristic of Bombus vestalis is the lack of corbiculae or a pollen basket on the hind legs of the bee. This is evolutionarily relevant because Bombus vestalis does not make its own nests or collect its own resources. With its larger body and poor foraging skills, the female Bombus vestalis are adapted to taking over nests of other species which have already been made and using the workers in these colonies to collect pollen.

A brood of bees successfully raised by Bombus vestalis is mainly female. This is evolutionarily beneficial since the female is the sex which takes over host species and allows Bombus vestalis to reproduce. In fact, many of the male bees which are hatched in a nest that has been taken over by Bombus vestalis are a result of host workers who were able to successfully lay eggs despite the aggression of the invaders. Although they display haplodiploidy where males are haploid and females are diploid, morphologically, male and female Bombus vestalis are very similar and can be classified with a mosaic gynander. They have similar mandible and body structures, and it is likely that the differences are only derived from chromosomal alterations during development of the embryo.

Bombus vestalis is part of the family Apidae. Until recently, it used to be identified as Psithryus vestalis. It was then renamed under the genus Bombus and classified as subgenus Psithyrus. The subgenus Psithyrus refers to a variety of cuckoo bumblebees who have no worker caste and reproduce by taking over host colonies and using workers of those colonies to rear their young. They can be confused with the bee Bombus bohemicus since both are found in similar regions and are parasitic in behavior. However, Bombus bohemicus is more intense in color and is a generalist parasite when choosing its host.

The bumblebee is common from North Africa in the south to southern Sweden in the north, and from Ireland in the west to northern Iran in the east. It is common throughout England and Wales, but has only been sighted in Scotland since 2009. Using data from the National Biodiversity Network gateway, Bombus vestalis is most prominent in south-east England.

In Ireland the bee was believed to be all but extinct in the 20th century with the last recorded sighting in 1926 in Carlow. However, in 2014 a population was rediscovered in the walled garden at Saint Enda's Park in Rathfarnham.