软体动物门（学名：Mollusca）属于无脊椎动物，就其物种多样性而言，是动物界的第二大门，仅次于节肢动物门，共有十万多种。软体动物能适应许多不同环境，分布广泛，从寒带、温带到热带，从海洋到河川、湖泊，从平原到高山，陆地、淡水和咸水中都有大量成员，例如蜗牛、河蚌、海螺、乌贼等物种。

软件动物型态、习性差异甚大，最大的软体动物大王乌贼的腕展开可达12公尺 ，最小的螺类却仅有1厘米长。但是牠们有共同的基本特征，身体无内骨骼且软，大多数不分节，身体结构可分为头、足、内脏团和外套膜4个部分。部分软件动物的外套膜会分泌出钙质的硬壳保护身体。外套模的形状因种类而不同。除了成年期的腹足动物之外，软件动物的的壳体都是左右对称的。

The molluscs or mollusks /ˈmɒləsks/ compose the large phylum of invertebrate animals known as the Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The scientific study of molluscs is called malacology.