Allopatric speciation (from Ancient Greek ἄλλος (állos) 'other', and πατρίς (patrís) 'fatherland') – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model^: 86 – is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

Various geographic changes can arise such as the movement of continents, and the formation of mountains, islands, bodies of water, or glaciers. Human activity such as agriculture or developments can also change the distribution of species populations. These factors can substantially alter a region's geography, resulting in the separation of a species population into isolated subpopulations. The vicariant populations then undergo genetic changes as they become subjected to different selective pressures, experience genetic drift, and accumulate different mutations in the separated populations' gene pools. The barriers prevent the exchange of genetic information between the two populations leading to reproductive isolation. If the two populations come into contact they will be unable to reproduce—effectively speciating. Other isolating factors such as population dispersal leading to emigration can cause speciation (for instance, the dispersal and isolation of a species on an oceanic island) and is considered a special case of allopatric speciation called peripatric speciation.

Allopatric speciation is typically subdivided into two major models: vicariance and peripatric. Both models differ from one another by virtue of their population sizes and geographic isolating mechanisms. The terms allopatry and vicariance are often used in biogeography to describe the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other—they do not occur together or only occur within a narrow zone of contact. Historically, the language used to refer to modes of speciation directly reflected biogeographical distributions. As such, allopatry is a geographical distribution opposed to sympatry (speciation within the same area). Furthermore, the terms allopatric, vicariant, and geographical speciation are often used interchangeably in the scientific literature. This article will follow a similar theme, with the exception of special cases such as peripatric, centrifugal, among others.

Observation of nature creates difficulties in witnessing allopatric speciation from "start-to-finish" as it operates as a dynamic process. From this arises a host of various issues in defining species, defining isolating barriers, measuring reproductive isolation, among others. Nevertheless, verbal and mathematical models, laboratory experiments, and empirical evidence overwhelmingly supports the occurrence of allopatric speciation in nature.^: 87–105 Mathematical modeling of the genetic basis of reproductive isolation supports the plausibility of allopatric speciation; whereas laboratory experiments of Drosophila and other animal and plant species have confirmed that reproductive isolation evolves as a byproduct of natural selection.^: 87