The Capitanian mass extinction event (also known as the end-Guadalupian extinction event, the Guadalupian-Lopingian boundary mass extinction, the pre-Lopingian crisis, or the Middle Permian extinction) was a major mass extinction event that occurred towards the end of the Capitanian stage and Guadalupian (Middle Permian) epoch of the Permian period. The mass extinction occurred during a period of decreased species richness and increased extinction rates. It is often called the end-Guadalupian extinction event because of its initial recognition between the Guadalupian and Lopingian series; however, more refined stratigraphic study suggests that extinction peaks in many taxonomic groups occurred within the Guadalupian, in the latter half of the Capitanian age. The extinction event has been argued to have begun around 262 million years ago with the Late Guadalupian crisis, though its most intense pulse occurred 259 million years ago in what is known as the Guadalupian-Lopingian boundary event.

Having historically been conflated with the more widely known end-Permian mass extinction event, and only having been recognised as a distinct extinction event beginning in 1994, this mass extinction is believed to be the third largest of the Phanerozoic in terms of the percentage of genera (33-35%) and species (60-63%) lost after the end-Permian and Late Ordovician mass extinction, respectively, while being the fifth worst in terms of ecological severity. The global nature of the Capitanian mass extinction has been called into question by some palaeontologists as a result of some analyses finding it to have affected only low-latitude taxa in the Northern Hemisphere.

In the aftermath of Olson's Extinction, global diversity rose during the Capitanian. This was probably the result of disaster taxa replacing extinct guilds. The Capitanian mass extinction greatly reduced disparity (the range of different guilds); eight guilds were lost. It impacted the diversity within individual communities more severely than the Permian–Triassic extinction event. Although faunas began recovery immediately after the Capitanian extinction event, rebuilding complex trophic structures and refilling guilds, diversity and disparity fell further until the Permian–Triassic boundary.

The impact of the Capitanian extinction event on marine ecosystems is still heavily debated by palaeontologists. Early estimates indicated a loss of marine invertebrate genera between 35 and 47%, while an estimate published in 2016 suggested a loss of 33–35% of marine genera when corrected for background extinction, the Signor–Lipps effect and clustering of extinctions in certain taxa. The loss of marine invertebrates during the Capitanian mass extinction was comparable in magnitude to the Cretaceous–Paleogene extinction event. Some studies have considered it the third or fourth greatest mass extinction in terms of the proportion of marine invertebrate genera lost; a different study found the Capitanian extinction event to be only the ninth worst in terms of taxonomic severity (number of genera lost) but found it to be the fifth worst with regard to its ecological impact (i.e., the degree of taxonomic restructuring within ecosystems or the loss of ecological niches or even entire ecosystems themselves).

Few published estimates for the impact on terrestrial ecosystems exist for the Capitanian mass extinction. Among vertebrates, Day and colleagues suggested a 74–80% loss of generic richness in tetrapods of the Karoo Basin in South Africa, including the extinction of the dinocephalians. In land plants, Stevens and colleagues found an extinction of 56% of plant species recorded in the mid-Upper Shihhotse Formation in North China, which was approximately mid-Capitanian in age. 24% of plant species in South China went extinct.

Although it is known that the Capitanian mass extinction occurred after Olson's Extinction and before the Permian–Triassic extinction event, the exact age of the Capitanian mass extinction remains controversial. Part of the reason for this is the poorly resolved biostratigraphy of the Guadalupian-Lopingian boundary interval. It is also partly due to the somewhat circumstantial age of the Capitanian–Wuchiapingian boundary itself, which is currently estimated to be approximately 259.1 million years old, but is subject to change by the Subcommission on Permian Stratigraphy of the International Commission on Stratigraphy. Additionally, there is a dispute regarding the severity of the extinction and whether the extinction in China happened at the same time as the extinction in Spitsbergen. According to one study, the Capitanian mass extinction was not one discrete event but a continuous decline in diversity that began at the end of the Wordian. Another study examining fossiliferous facies in Svalbard found no evidence for a sudden mass extinction, instead attributing local biotic changes during the Capitanian to the southward migration of many taxa through the Zechstein Sea. Carbonate platform deposits in Hungary and Hydra show no sign of an extinction event at the end of the Capitanian; the extinction event there is recorded in the middle Capitanian.

The volcanics of the Emeishan Traps, which are interbedded with tropical carbonate platforms of the Maokou Formation, are unique for preserving a mass extinction and the cause of that mass extinction. Large phreatomagmatic eruptions occurred when the Emeishan Traps first started to erupt, leading to the extinction of fusulinacean foraminifera and calcareous algae.

In the absence of radiometric ages directly constraining the extinction horizons themselves in the marine sections, most recent studies refrain from placing a number on its age, but based on extrapolations from the Permian timescale an age of approximately 260–262 Ma has been estimated; this fits broadly with radiometric ages from the terrestrial realm, assuming the two events are contemporaneous. Plant losses occurred either at the same time as the marine extinction or after it.