The Pyrenean Mountain Dog, known as the Great Pyrenees in North America, is a large breed of dog used as a livestock guardian dog. It should not be confused with the Pyrenean Mastiff.

As late as 1874 the breed was not completely standardized in appearance, with two major sub-types recorded, the Western and the Eastern. They are related to several other large white European livestock guardian dogs (LGD), including the Italian Maremma Sheepdog, Kuvasz (Hungary), Akbash Dog (Turkey) and Polish Tatra or Polski Owczarek Podhalański, and somewhat less closely to the Newfoundland and St. Bernard. According to the Great Pyrenees Club of America, the Great Pyrenees is naturally nocturnal and aggressive with any predators that may harm its flock. However, the breed can typically be trusted with small, young, and helpless animals of any kind due to its natural guardian instinct.