The tricolored bat (Perimyotis subflavus) or American perimyotis is a species of microbat native to eastern North America. Formerly known as the eastern pipistrelle, based on the incorrect belief that it was closely related to European Pipistrellus species, the closest known relative of the tricolored bat is now recognized as the canyon bat. Its common name "tricolored bat" derives from the coloration of the hairs on its back, which have three distinct color bands. It is the smallest bat species in the eastern and midwestern US, with individuals weighing only 4.6–7.9 g (0.16–0.28 oz). This species mates in the fall before hibernation, though due to sperm storage, females do not become pregnant until the spring. Young are born helpless, though rapidly develop, flying and foraging for themselves by four weeks old. It has a relatively long lifespan, and can live nearly fifteen years.

In the summer, females roost in small groups and males roost solitarily in tree foliage or beard lichen. It eats a diverse array of insects, foraging with a slow, erratic flight and navigating via echolocation. Though once considered one of the most common bat species in its range, its populations have declined rapidly since 2006 with the introduction of the fungal disease white-nose syndrome. It was listed as an endangered species in 2012 in Canada, and has been petitioned for inclusion on the US endangered species list. Along with the silver-haired bat, the tricolored bat is one of two bat species whose rabies variants have most frequently been implicated in human rabies deaths in the US, with sixteen deaths from 1958–2000.

The tricolored bat was described as a new species in 1832 by French zoologist Frédéric Cuvier, who placed it in the genus Vespertilio with a scientific name of Vespertilio subflavus. The holotype had been collected in the US state of Georgia by American naturalist John Eatton Le Conte. In 1897, American zoologist Gerrit Smith Miller Jr. placed it in the genus Pipistrellus for the first time as a result of its physical similarities to other members of the genus. Its common name was thus the "eastern pipistrelle". In 1984 its status was reviewed, with H. Menu concluding that it was more similar in appearance to species in the genus Myotis rather than Pipistrellus, erecting a new genus for the taxon: Perimyotis. The name "Perimyotis" means "around Myotis", alluding to its similarities with the genus. This classification was not immediately accepted, however, with a 1985 study instead grouping it and the canyon bat into the genus Parastrellus and a 1987 publication maintaining that the tricolored bat was part of the Pipistrellus genus, though they listed Perimyotis as one of seven subgenera. However, in 2003, a genetic study concluded that the tricolored bat was distinct from Pipistrellus species (and the canyon bat), confirming the validity of the genus Perimyotis. Usage of the scientific name Perimyotis subflavus is widely accepted as of 2019. Its common name was changed from "eastern pipistrelle" to "tricolored bat" to reflect its revised classification.

A 2010 study found that the sister taxon of the tricolored bat is the canyon bat, with the authors suggesting that these two genera form a tribe, though declining to name and formally describe the tribe. Instead, they referred to it as the "perimyotine group", which they gave as the most basal member of a clade that also included the following tribes: Nycticeiini, Eptesicini, Vespertilioni, and another unnamed tribe referred to as the "hysugine group" (including Chalinolobus, Hypsugo, Laephotis, Neoromicia, Nycticeinops, Tylonycteris, and Vespadelus). The tribe was later described as Perimyotini.

Its species name "subflavus" means "a little less than yellow", meaning yellowish or pale yellow. Four subspecies are recognized:

The tricolored bat has blond fur that is distinctly tricolored on its back. Individual hairs tricolored: dark gray at the base, buffy (yellowish brown) in the middle, and brown or reddish brown at the tip. It is the smallest bat species found in the eastern and midwestern US. Individuals have a forearm length of 31.4–34.1 mm (1.24–1.34 in) and weigh 4.6–7.9 g (0.16–0.28 oz). Its forearms are reddish, contrasting sharply with the black membranes of its wings. Its wingspan is 21–26 cm (8.3–10.2 in). It has a dental formula of 2.1.2.33.1.2.3 for a total of 34 teeth.

The tricolored bat is a seasonal breeder, with copulation (mating) occurring in the fall before hibernation. Ovulation does not occur until the spring, however, and females store the males' sperm in their uteruses through the winter. Gestation (pregnancy) length is about forty-four days, with females giving birth in June or July. The litter size is typically two individuals. As newborns, the combined mass of twin pups can be as great as 58% of the mother's postpartum mass. At birth, the young lack fur and their eyes are closed. The mother leaves the offspring behind at the roost while she forages at night. Offspring develop rapidly, beginning to fly at three weeks old. By four weeks old, they are foraging for themselves. Young do not reach sexual maturity in their first fall; they do not breed until their second fall.

Individuals can live at least 14.8 years in the wild; juveniles have higher mortality rates than adults, and females have higher mortality rates than males.