A longdog (also long-dog or long dog) is any dog of sighthound type, whether pure-bred or not. It is usually a cross-breed between two sighthounds of different breeds, one of which is usually a Greyhound.

It is distinct from the lurcher, which is a cross between a sighthound and a working dog, usually a terrier or herding dog. A longdog is generally larger than a feist, which is an American cross.