The Baikal seal, Lake Baikal seal or nerpa (Pusa sibirica), is a species of earless seal endemic to Lake Baikal in Siberia, Russia. Like the Caspian seal, it is related to the Arctic ringed seal. The Baikal seal is one of the smallest true seals and the only exclusively freshwater pinniped species. A subpopulation of inland harbour seals living in the Hudson Bay region of Quebec, Canada (Lacs des Loups Marins harbour seals), as well as the Saimaa ringed seal and the Ladoga seal (both ringed seal subspecies), are also found in fresh water, but these seals are part of species that also have marine populations.

The most recent population estimates are 80,000 to 100,000 animals, roughly equaling the expected carrying capacity of the lake. At present, the species is not considered threatened.

The Baikal seal is one of the smallest true seals. Adults typically grow to 1.1–1.4 m (3 ft 7 in – 4 ft 7 in) in length with a body mass from 63 to 70 kg (139 to 154 lb).[3] The maximum reported size is 1.65 m (5 ft 5 in) in length and 130 kg (290 lb) in weight. There are significant annual variations in the weight, with lowest weight in the spring and highest weight, about 38–42% more, in the fall. The animals show very little sexual dimorphism; males are only slightly larger than females. They have a uniform, steely-grey coat on their backs and fur with a yellowish tinge on their abdomens. As the coat weathers, it becomes brownish. When born, the pups weigh 3–3.5 kg (6.6–7.7 lb) and are about 70 cm (2 ft 4 in) long. They have coats of white, silky, natal fur. This fur is quickly shed and exchanged for a darker coat, much like that of adults. Rarely, Baikal seals can be found with spotted coats.

The Baikal seal lives only in the waters of Lake Baikal. It is something of a mystery how Baikal seals came to live there in the first place. The skull structure of the Baikal seal suggests it is closely related to the Caspian seal. In addition, the morphological structures in both species suggest they are descended from Arctic ringed seals. They may have swum up rivers and streams or possibly Lake Baikal was linked to the ocean at some point through a large body of water, such as the Tethys Ocean, West Siberian Glacial Lake or West Siberian Plain, formed in a previous ice age. The seals are estimated to have inhabited Lake Baikal for some two million years.

The areas of the lake in which the Baikal seals reside change depending on the season, as well as other environmental factors. They are solitary animals for the majority of the year, sometimes living kilometres away from other Baikal seals. In general, a higher concentration of Baikal seals is found in the northern parts of the lake, because the longer winter keeps the ice frozen longer, which is preferable for pupping. However, in recent years, migrations to the southern half of the lake have occurred, possibly to evade hunters. In winter, when the lake is frozen over, seals maintain a few breathing holes over a given area and tend to remain nearby, not interfering with the food supplies of nearby seals. When the ice begins to melt, Baikal seals tend to keep to the shoreline.

Since 2008, the Baikal seal has been listed as a Least Concern species on the IUCN Red List. This means that they are not currently threatened or endangered. In 1994, the Russian government estimated that they numbered 104,000. In 2000, Greenpeace performed its own count and found an estimated 55,000 to 65,000 seals. The most recent estimates are 80,000-100,000 animals, roughly equaling the carrying capacity of the lake.

In the last century, the kill quota for hunting Baikal seals was raised several times, most notably after the fur industry boomed in the late 1970s and when official counts began indicating more Baikal seals were present than previously known. The quota in 1999, 6,000, was lowered in 2000 to 3,500, which was still nearly 5% of the population if the Greenpeace count is correct. In 2013–2014, the hunting quota was set at 2,500. In addition, new techniques, such as netting breathing holes and seal dens to catch pups, have been introduced. In 2001, a prime seal pelt would bring 1,000 rubles at market. In 2004–2006, about 2,000 seals were killed per year according to official Russian statistics, but in the same period another 1,500–4,000 are thought to have died annually due to drowning in fishing gear, poaching, and the like. In 2012–2013, it was estimated that 2,300–2,800 were hunted per year (combined legal hunting and poaching). Some groups have pressured for higher hunting quotas.

Another problem at Lake Baikal is the introduction of pollutants into the ecosystem. Pesticides such as DDT and hexachlorocyclohexane, as well as industrial waste, mainly from the Baikalisk pulp and paper plant, are thought to have exacerbated several disease epidemics among Baikal seal populations. The chemicals are speculated to concentrate up the food chain and weaken the Baikal seal's immune system, making them susceptible to diseases such as canine distemper and the plague, which was the cause of a serious Baikal seal epidemic that resulted in the deaths of 5,000–6,500 animals in 1987–1988. Small numbers died as recently as 2000, but the reason for their deaths is unclear. Canine distemper is still present in the Baikal seal population, but has not caused mass deaths since the earlier outbreaks. In general, levels of DDT and non-ortho PCB have declined in the lake from the 1990s, levels of mono-ortho PCB are unchanged, and the level of perfluorochemicals have increased. Industrialization of the area near Lake Baikal is increasing and future monitoring is necessary. At present, Baikal seals show lower levels of contaminants than seals of Europe and North America, but higher than those in the Arctic.

The most serious future threat to the survival of the seal may be global warming, which has the potential to seriously affect a closed cold-water ecosystem such as that of Lake Baikal.

The only known natural predator of adult Baikal seals is the brown bear, but this is not believed to occur frequently. The seal pups are typically hidden in a den, but can fall prey to smaller land predators such as the red fox, the sable and the white-tailed eagle.