The Sealyham Terrier (Welsh: Daeargi Sealyham) is a rare Welsh breed of small to medium-sized terrier that originated in Wales as a working dog. It is principally a white-bodied, rough-coated breed, developed in the mid-to-late-19th century by Captain John Edwardes at Sealyham House, Pembrokeshire.

Following the First World War, it surged in popularity and was associated with Hollywood stars and members of the British royal family. Its numbers have dropped significantly since then, with the breed listed as a Vulnerable Native Breed by the Kennel Club; an all-time low was recorded in 2008 when only 43 puppies were registered in the United Kingdom. This decline has been blamed on an influx of foreign and designer breeds, and the Sealyham's reduced usefulness as a working dog.

This breed is equally suitable as a family dog or a working terrier, given the right training. It is affected by few breed specific breed disorders, with the only two prevalent conditions being lens luxation and canine degenerative myelopathy. A DNA test is now readily available to identify dogs who carry the gene that causes lens luxation and breeding programs can be adjusted.