The Sealyham Terrier has as its origin the region of Sealyham, in Pembrokeshire, Wales. It is a cross of several different terriers and hounds, among them the Bull and the Dandie Dinmont and the Fox terriers and the Westy. Another important part of its genetic makeup is the Flandres Basset Hound. From the Basset the Sealyham acquired the short carriage and the heavy duty front legs capable of digging out burrows quickly and efficiently.

The Sealyham Terrier is a terrier which was custom designed by Captain John Tucker Edwardes, an Otter Hound enthusiast. Captain Edwardes desired to produce a dog which would be agile enough to rout out the Otters from between the rocks and crevices in which they hid , into his waiting pack of Otter hounds. Besides, he reasoned, the landowners of the Wales countryside needed a dog that could more easily get into the burrows of the animals which killed their chickens and destroyed their grain stores. His Sealyham Terrier fit the bill, being speedy enough to run with the hounds and small and agile enough to go to ground and less "scrappy" in nature so that he got along well with the pack. This terrier is by nature more of a pack dog than many terriers , this is perhaps due to the genetic addition of the Basset in the gene pool. Whatever the reason, this terrier, although "game" when it comes to the hunt, is more of a couch potato than other terrier types and is noted for being laid back.

The Sealyham is a small dog about 12 inches at the shoulder, with an appearance of stockiness to his body. The coat is the typical wire coat, being white with patches of badger on the head and ears. It is long and should be hand stripped, although pet owners are more likely to take it to the groomer and have it clipped. He is a dog whose form exemplifies his function. The body is deep chested and the back perfectly straight and while the front legs are short the hind legs have a well bent stifle. In layman's terms this type of angulation means that the dog is exceptionally agile and suited to the type of terrain that is rough and rocky. It also means that he has endurance and speed, both necessary features for a dog meant to run the otter to the ground. Temperamentally he is a persistent dog and not likely to give in, stubbornness is a trait that can be difficult when it comes to training but is an excellent quality for a dog that hunts.

In modern day, this terrier is not such a hunter and has adapted himself well to civilized apartment life. He is gentle and affectionate and also adaptable. He prefers the indoor life and makes a great city dog, a short walk will suffice for exercise and he is quite happy spending the rest of his time with the family. If he is brought up with other dogs in the household, he gets along well with them.

The Sealyham is very popular as a show dog, having been registered with both the American kennel club and the British Kennel club since 1910.