The Guernsey is a small, cream-and-brown breed of dairy cattle, renowned for the high butterfat content of its milk, as well as its hardiness and genial disposition.

As its name implies, the Guernsey was bred on the British Channel Island of Guernsey. It descended from cattle stock brought over from nearby Normandy, and was first recorded as a separate breed around 1700. In 1789, imports of foreign cattle into Guernsey were forbidden by law to maintain the purity of the breed, although exports of cattle and semen were for a while an important economic resource for the island.

With an average weight slightly greater than the 900 pounds (450 kg) of the Jersey cow, Guernsey cows are small, producing more milk per unit of body weight than any other breed. Bulls are also small by standards of domestic cattle, but can be surprisingly aggressive. They will dominate the more docile Jerseys, if inadvertently allowed to mix.

Overseas farmers who preferred maximum quality, immunity to disease and high milk production selected Guernseys; the breed is well-established in the United States, Canada, and elsewhere.