The Eurasian brown bear (Ursus arctos arctos) is one of the most common subspecies of the brown bear, and is found in much of Eurasia. It is also called the European brown bear, common brown bear, common bear, European bear, and colloquially by many other names. The genetic diversity of present-day brown bears (Ursus arctos) has been extensively studied over the years and appears to be geographically structured into five main clades based upon analysis of the mtDNA.

The Eurasian brown bear has brown fur, which ranges from yellowish-brown to dark brown, red-brown, and almost black in some cases; albinism has also been recorded. The fur is dense to varying degrees and the hair can grow up to 10 cm (3.9 in) in length. The head normally is quite round and has relatively small rounded ears, a wide skull, and a mouth equipped with 42 teeth, including predatory teeth. It has a powerful bone structure and large paws equipped with claws that can grow up to 10 cm (3.9 in) in length. The weight varies depending on habitat and the time of the year. A full-grown male weighs on average between 350 and 500 kg and reaches a maximum weight of 650 kg and length of nearly 2.5 m (8.2 ft). Females typically range between 150 and 300 kg and reach a maximum weight of 450 kg . They have a lifespan of 20 to 30 years in the wild.

Eurasian brown bears were used in Ancient Rome for fighting in arenas. The strongest bears apparently came from Caledonia and Dalmatia.

In antiquity, the Eurasian brown bear was largely carnivorous, with 80% of its diet consisting of animal matter. However, as its habitat increasingly diminished, the portion of meat in its diet decreased with it until by the late Middle Ages, meat consisted of only 40% of its dietary intake. Today, meat makes up little more than 10–15% of its diet. Whenever possible, the brown bear will consume sheep.

Unlike in North America, where an average of two people a year are killed by bears, Scandinavia only has records of three fatal bear attacks within the last century. In late 2019, brown bears killed three men in Romania in just over a month.

The oldest fossils are from the Choukoutien, China, and date back about 500,000 years. It is known from mtDNA studies that during the Pleistocene ice age it was too cold for the brown bear to survive in Europe except in three places: Russia, Spain, and the Balkans. However, a newer study found that brown bears were present in France and Belgium during the Last Glacial Maximum as well, indicating they were not as restricted to southern refugia as previously thought.

Modern research has made it possible to track the origin of the subspecies. The species to which it belongs developed more than 500,000 years ago, and researchers have found that the Eurasian brown bear separated about 850,000 years ago, with one branch based in Western Europe and the other branch in Russia, Eastern Europe and Asia. Through the research of mitochondrial DNA (mtDNA), researchers have found that the European family has divided into two clades—one in the Iberian Peninsula and the Balkans, the other in Russia.

There is a population in Scandinavia that includes bears of the western and eastern lineages. By analyzing the mtDNA of the southern population, researchers have found that they have probably come from populations in the Pyrenees in Southern France and Spain and the Cantabrian Mountains (Spain). Bears from these populations spread to southern Scandinavia after the last ice age. The northern bear populations originate in the Finnish/Russian population. Probably their ancestors survived the ice age in the ice-free areas west of the Ural Mountains, and thereafter spread to Northern Europe.