The Cavalier King Charles Spaniel (CKCS) is a British breed of toy dog of spaniel type. Four colours are recognised: Blenheim (chestnut and white), tricolour (black/white/tan), black and tan, and ruby; the coat is smooth and silky. The lifespan is usually between eight and twelve years.

The Cavalier King Charles changed dramatically in the late seventeenth century, when it was inter-bred with flat-nosed breeds. Until the 1920s, it shared the same history as the smaller King Charles Spaniel. Breeders attempted to recreate what they considered to be the original configuration – a dog resembling Charles II's spaniel of the English Civil War period, when supporters of the king were known as Cavaliers.

During the early part of the 18th century, John Churchill, 1st Duke of Marlborough, kept red and white King Charles type spaniels for hunting. The duke recorded that they were able to keep up with a trotting horse. His estate was named Blenheim in honour of his victory at the Battle of Blenheim. Because of this influence, the red and white variety of the King Charles Spaniel and thus the Cavalier King Charles Spaniel became known as the Blenheim.

Attempts were made to recreate the original Cavalier King Charles Spaniel as early as the turn of the 20th century, using the now extinct Toy Trawler Spaniels. These attempts were documented by Judith Blunt-Lytton, 16th Baroness Wentworth, in the book "Toy Dogs and Their Ancestors Including the History And Management of Toy Spaniels, Pekingese, Japanese and Pomeranians" published under the name of the "Hon. Mrs. Neville Lytton" in 1911.

In 1926, the American Roswell Eldridge offered a dog show class prize of 25 pounds of sterling silver each as a prize for the best male and females of "Blenheim Spaniels of the old type, as shown in pictures of Charles II of England's time, long face, no stop, flat skull, not inclined to be domed, with spot in centre of skull." The breeders of the era were appalled, although several entered what they considered to be sub-par Cavalier King Charles Spaniels in the competition. Eldridge died before seeing his plan come to fruition, but several breeders believed in what he said and in 1928 the first Cavalier club was formed. The first standard was created, based on a dog named "Ann's Son" owned by Mostyn Walker, and the Kennel Club recognised the variety as "King Charles Spaniels, Cavalier type".

The Second World War caused a drastic setback to the emerging breed, with the vast majority of breeding stock destroyed because of wartime hardship and food shortages. For instance, in the Ttiweh Cavalier Kennel, the population of 60 dogs dropped to three during the 1940s. Following the war, few dogs survived as the new beginning from which all present-day Cavaliers descend. And in 1945, the Kennel Club first recognised the breed in its own right as the Cavalier King Charles Spaniel.

The first recorded Cavalier living in the United States was brought from the United Kingdom in 1956 by Lyons and Sally Brown who, together with Elizabeth Spalding and other enthusiasts, founded the Cavalier King Charles Club USA that continues to the present day. In 1994, the American Cavalier King Charles Spaniel Club was created by a group of breeders to apply for recognition by the American Kennel Club. It was recognised by the American Kennel Club in 1995, and the ACKCSC became the parent club for Cavaliers. Since 2000, it has grown in popularity in the United States and ranks as the 13th most popular pure-breed in the United States.

Historically the Cavalier King Charles Spaniel was a lap dog and is small for a spaniel, with fully grown adults comparable in size to adolescents of other larger spaniel breeds. The tail is usually not docked, and the Cavalier should have a silky coat of moderate length. Standards state that it should be free from curl, although a slight wave is allowed. Feathering can grow on their ears, feet, legs and tail in adulthood. Standards require this be kept long, with the feathering on the feet a particularly important aspect of the breed's features.