Tortoiseshell is a cat coat coloring named for its similarity to tortoiseshell pattern. Like tortoiseshell-and-white or calico cats, tortoiseshell cats are almost exclusively female. Male tortoiseshells are rare and are usually sterile.

Tortoiseshell or tortoise shell is a material produced from the shells of the larger species of tortoise and turtle, mainly the hawksbill sea turtle, which is a critically endangered species according to the IUCN Red List largely because of its exploitation for this trade. The large size, fine color, and unusual form of the hawksbill's scutes make it especially suitable. The distinctive patterning is referred to in names such as the tortoiseshell cat, several breeds of guinea pig, and the common names of several species of the butterfly genera Nymphalis and Aglais, and some other uses.