The Sgian Dubh (pronounced "skee(a)n doo", /ski:n du:/, or lightly diphthongised /ski?n/) is a ceremonial dagger (Gaelic sgian) worn as part of the modern Scottish Highland dress along with the kilt. It is worn tucked into the hose with only the pommel visible.

The name comes from the Gaelic meaning "black knife", where "black" may refer to the usual colour of the handle of the knife. It is also suggested that "black" means secret, or hidden, as in the word blackmail. This is based on the stories and theories surrounding the knife's origin and the meaning of "Dubh" in Gaelic.

The sgian dubh may have evolved from the sgian achlais, a dagger that could be concealed under the armpit. Used by the Scots of the 17th and 18th centuries, this knife was slightly larger than the average modern sgian dubh and was carried in the upper sleeve or lining of the body of the jacket.

Courtesy and etiquette would demand that when entering the home of a friend, any concealed weapons would be revealed. It follows that the sgian achlais would be removed from its hiding-place and displayed in the stocking top held securely by the garters.

The sgian dubh also resembles the small skinning knife that is part of the typical set of hunting knives. These sets contain a butchering knife with a 9-10 inch blade, and a skinner with a blade of about 4 inches. These knives usually had antler handles, as do many early sgian dubhs. The larger knife is likely the ancestor of the modern dirk.