The St. Bernard or Saint Bernard (UK: /ˈbɜːrnərd/, US: /bərˈnɑːrd/) is a breed of very large working dog from the Western Alps in France, Italy and Switzerland. They were originally bred for rescue work by the hospice of the Great St Bernard Pass on the Italian-Swiss border. The hospice, built by and named after the Alpine monk Saint Bernard of Menthon, acquired its first dogs between 1660 and 1670. The breed has become famous through tales of Alpine rescues, as well as for its large size and gentle temperament.

The St. Bernard is recognized internationally today as one of the molosser breeds. It is a giant dog. The coat is smooth being close and flat. The colour is primarily white with smaller or larger red patches with a clear to dark red mantle and reddish-brown mantle being the most 'valuable'. A brindle reddish colour and brownish-yellow colour are both allowed but less desirable. The tail is long and heavy, hanging high. The eye colour ranges from light brown to dark brown and should have naturally tight lids, with haws only slightly visible.

The earliest written records of the St. Bernard are from monks at the Great St Bernard Hospice at the Great St Bernard Pass in 1707, with paintings and drawings of the dog dating even earlier. Early British accounts of the breed described the breed as the Alpine Spaniel. The first evidence that the dogs were in use at the monastery is in two paintings dating to 1690 by Italian artist Salvator Rosa. The most famous St. Bernard to save people at the pass was Barry (sometimes spelled Berry), who reportedly saved somewhere between 40 and 100 lives. There is a monument to Barry in the Cimetière des Chiens, and his body was preserved in the Natural History Museum in Bern.

Another famous dog was Rutor, the faithful companion of the Italian priest Pierre Chanoux, named after the peak Tête du Rutor located above the Little St. Bernard pass. The classic St. Bernard looked very different from the St. Bernard of today because of crossbreeding. Severe winters from 1816 to 1818 led to increased numbers of avalanches, killing many of the dogs used for breeding while they were performing rescues. In an attempt to preserve the breed, the remaining St. Bernards were crossed with Newfoundlands brought from the Colony of Newfoundland in the 1850s, as well as with other breeds. This led to a significant change in their appearance and abilities. The long fur they inherited would freeze in the snowy climate of the Alps, weighing them down and reducing their effectiveness as rescue dogs.

The dogs never received any special training from the monks. Instead, younger dogs would learn how to perform search and rescue operations from older dogs.

The Swiss St. Bernard Club was founded in Basel on 15 March 1884. The St. Bernard was the first breed entered into the Swiss Stud Book in 1884, and the breed standard was finally approved in 1888. Since then, the breed has been a Swiss national dog.

During World War I St. Bernard dogs assisted the Red Cross and were used to carry supplies to troops in the Italian mountains, stationed in places inaccessible to mules and horses.

The dogs at the Great St Bernard Hospice were working dogs that were smaller than today's show St. Bernards. Originally about the size of a German Shepherd Dog, the St. Bernard grew to the size of today's dog as kennel clubs and dog shows emphasized appearance over the dog's working ability, along with a closed stud book.