The Siberian tiger (Panthera tigris altaica), also known as the Amur tiger, Manchurian tiger, or Ussuri tiger, is a rare subspecies of tiger (P. tigris) confined completely to the Amur region in the Far East, where it is now protected. It is considered to be the largest of the nine recent tiger subspecies and the largest member of the family Felidae. The Siberian tiger is endangered. In the early 1900s, it lived throughout northeastern China, the Korean Peninsula, northeastern Mongolia and southeastern Russia. Today, the majority of the population is confined to a tiny part of Russia's southern Far East: the Amur-Ussuri region of Primorsky and Khabarovsky Krai. There are very few tigers in northeastern China and fewer still in North Korea. The South Korean population died out in 1922. By the 1940s the estimated population was down to fewer than 50 in the Russian Far East, although some hundreds still populated neighbouring China. The number increased to more than 200 in 1982, although in China there are now thought to be no more than a dozen or so Siberian tigers. Poaching has been brought under better control by frequent road inspections. Captive breeding and conservation programs are active. The Hengdaohezi Feline Breeding Centre in the northern Heilongjiang province of China,together with its partner Heilongjiang Siberian Tiger Park in Harbin, plan to release 620 Siberian tigers after its numbers have increased from 708 to 750. A 1996 count reported 430 Siberian tigers in the wild. However, Russian conservation efforts have led to a slight increase, or at least to a stable population of the subspecies, as the number of individuals in the Siberian forests was estimated to be between 431 and 529 in 2005. According to the World Wide Fund for Nature, the latest Russian Census reports put this number to be anywhere between 480 and 520 without including the small numbers of this subspecies present in mainland China.

The Siberian tiger is typically 2-4 inches taller at the head than the Bengal tiger, which is about 107-110 cm (42-43 in) tall. Mature males reach an average head and body length of 190-220 centimetres (75-97 in). The largest male with largely assured references was 350 cm (138 in) "over curves" (3,30 m/130 in. between pegs) in total length. The tail length in fully grown males is about 1 m (39 in). Weights of up to 318 kg (700 lb) have been recorded and exceptionally large males weighing up to 384 kg (847 lb) are mentioned in the literature but, according to Mazak, none of these cases can be confirmed via reliable sources. A further unconfirmed report tells of a male tiger shot in the Sikhote Alin Mountains in 1950 weighing 384.8 kg (846.6 lb) and measuring 3.48 m (11.5 ft). Females are normally smaller than males and weigh 100-167 kg (220-368 lb), probably up to 180 kg (400 lb). The "Siberian Tiger Project", which has operated from Sikhote-Alin Zapovednik since 1992, found that 215 kg (474 lb) seemed to be the largest that they were able to verify, albeit from a limited number of specimens. According to modern research of wild Siberian tigers in Sikhote-Aline, an average adult male tiger (>35 months) weighs 167.3 - 185.7 kg (the average asymptotic limit, computed by use of the Michaelis-Menten formula, gives 222.3 kg for male tigers) and an adult tigress – 117.9 – 122.6 kg, respectively.

The mean weight of historical Siberian tigers is supposed to be higher: 215.3-260 kg for male tigers. At least one authority suspects that this is the difference between real weights and hunter's estimates. Dale Miquelle, program director of the Siberian Tiger Project, writes that, despite repeated claims in the popular literature that the Siberian is the largest of all tigers, their measurements on more than fifty captured individuals suggest that body size is, in fact, similar to that of Bengal tigers. Apart from its size, the Siberian tiger is differentiated from other tiger subspecies by its mane of fur around the neck, which is much more developed than in other subspecies as an adaptation against the cold. The fur of this subspecies grows longer and thicker than that of other tigers. During cold winter months, the fur can measure as long as 21 inches with 3,000 hairs over every square centimetre of its surface. The paws have extra fur to provide insulation against the snow. Siberian tigers have more white in their coats than other subspecies and coat colour is more gold than orange. Compared to other subspecies, the Siberian tiger has less striping, the stripes being more brown than black. Stripes appear largely absent on the outer area of the front legs.