螺壳是指软体动物门腹足纲如蜗牛等动物的外壳，属于一种外骨骼。其作用不仅仅是肌肉的附着物，还有着保护软弱的躯体免受捕食动物或者其它外力的伤害。陆生蜗牛的壳还有屏蔽阳光，免于水分蒸发的作用。螺壳属于贝壳的一种，研究贝壳的学科叫做贝壳学。

绝大多数的螺壳都是成螺旋状，并且其螺旋都是右旋的:从壳顶往下看，旋转方向为顺时针。不过也有某些少数类目的螺壳是左旋的:旋转方向为顺时针。同时存在两种情况的物种，则更为罕见。

螺壳分为多层，其成分通常主要是碳酸钙。这些碳酸钙由软体动物的外套膜所分泌，在沉积到同样由外套膜所分泌的，被称为贝壳素的有机基质上，就变成了坚硬的外壳了。

The gastropod shell is part of the body of a gastropod or snail, a kind of mollusk or mollusc. The shell is an external skeleton or exoskeleton, which may serve for muscle attachment, but also for protection from predators, mechanical damage, dehydration, and calcium storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group. An excellent source for terminology of the gastropod shell is "How to Know the Eastern Land Snails" by John B. Burch now freely available at the Hathi Trust Digital Library.