The male bear, which weighs 205 kg (450 pounds) and is around 10-years-old, was released in the Alt Pirineu Natural Park, the largest in Spain's northestern Catalonia region on Monday, the regional government of Catalonia said in a statement.
The bear, named Goiat, arrived in Spain by road from a reserve in Slovenia, which has a thriving bear population and has helped to increase bear populations around Europe.
Native Pyrenean brown bears were almost extinct in the 1990s due largely to hunting and the first attempt to re-introduce them was in 1996-97 when three bears - two females and a male named Pyros - were brought from Slovenia.
There are now believed to be 35 brown bears on both sides of the Pyrenees mountain range which straddles the Spanish-French border, the majority of them the offspring of Pyros who is now 26-years-old and showing signs of ageing.
Wildlife officials wanted to introduce a new male bear onto Pyros's domain, in the name of genetic diversity.
Excessive inbreeding reduces genetic variation and, over time, leaves an animal population more vulnerable to diseases and deformities, specialists say.
The programme to reintroduce brown bears in the region has been strongly opposed by cattle and sheep farmers who say the bears are unwelcome and kill their livestock.
Brown bears originated about 600,000 years ago in China and were first reported in Europe about 250,000 years ago. They are the same species - ursus arctos - as the American grizzly.