Cuttlebone, also known as cuttlefish bone, is a hard, brittle internal structure (an internal shell) found in all members of the family Sepiidae, commonly known as cuttlefish, within the cephalopods. In other cephalopod families it is called a gladius.

Cuttlebone is composed primarily of aragonite. It is a chambered structure that the animal can fill with gas or liquid for buoyancy control. On the ventral (bottom) side of the cuttlebone is the highly modified siphuncle; this is the organ with which the cuttlebone is filled with gas or liquid. The microscopic structure of cuttlebone consists of narrow layers connected by numerous upright pillars.

Depending on the species, cuttlebones implode at a depth of 200 to 600 metres (660 to 1,970 ft). Because of this limitation, most species of cuttlefish live on the seafloor in shallow water, usually on a continental shelf.

When the cuttlefish dies, its body decomposes, leaving only the cuttlebone, which often washes up on beaches.

In the past, cuttlebones were ground up to make polishing powder, which was used by goldsmiths. The powder was also added to toothpaste, and was used as an antacid for medicinal purposes or as an absorbent. They were also used as an artistic carving medium during the 19th and 20th centuries.

Today, cuttlebones are commonly used as calcium-rich dietary supplements for caged birds, chinchillas, hermit crabs, reptiles, shrimp, and snails. These are not intended for human consumption.

As a carbonate-rich biogenic raw material, cuttlebone has potential to be used in the production of calcitic lime.

Because cuttlebone is able to withstand high temperatures and is easily carved, it serves as mold-making material for small metal castings for the creation of jewelry and small sculptural objects.