无板纲（学名：Aplacophora）是软体动物门的一个纲。仅有250种左右。是一类身体呈蠕虫状的软体动物。

过往当生物分类学纯綷以形态学为分类准则之时，无板纲物种曾被以为跟海参同类，这一点可在1987年之前的分类中看见。从1987年起，本分类得到确认为软件动物之一，因而成为一个独立的纲。当时本纲由两个群组成，分别为：

这两个群最初以为只是并系群，但近年的分子数据支持两个群是单系群的论说。

从分子数据及化石的分析，无板纲很大可能是从多板纲演化，只是后来这些「板」都退化了。

Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world. All known modern forms are shell-less: only some extinct primitive forms possessed valves. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear.