外骨骼是节肢动物门外壳的俗称，因为坚硬有如骨骼，因此得名。

节肢动物的体表覆盖着坚硬的体壁。体壁由三部分组成：表皮细胞层，基膜和角质层。表皮细胞层由一层活细胞组成，它向内分泌形成一层薄膜，叫做基膜，向外分泌形成厚的角质层。角质层除了能防止体内水分蒸发和保护内部构造外，还能与内壁所附着的肌肉共同完成各种运动，跟脊椎动物体内的骨骼有相似的作用，因此被叫做外骨骼。但节肢动物的"骨骼"并不是由钙所组成，他们是由甲壳素（又名几丁质），由N-乙酰-D-胺基葡萄糖化合而成，约由8000个单糖组成。因为这些节肢动物门的动物，"骨骼"位于身体外部，生长受限，所以体型都不会太大。

An exoskeleton (from Greek έξω, éxō "outer" and σκελετός, skeletos "skeleton") is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of certain sponges and the various groups of shelled mollusks, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons.