在植物命名中，种下分类群（infraspecific taxon）是在层级之下的一个物种分类单元。「换句话说，在单一物种的范围内，之后的等于是下一级」（i.e. within a species, infra = below）。种下分类群使用于三名法中。

In botany, an infraspecific name is the scientific name for any taxon below the rank of species, i.e. an infraspecific taxon. (A "taxon", plural "taxa", is a group of organisms to be given a particular name.) The scientific names of botanical taxa are regulated by the International Code of Nomenclature for algae, fungi, and plants (ICN). This specifies a 'three part name' for infraspecific taxa, plus a 'connecting term' to indicate the rank of the name. An example of such a name is Astrophytum myriostigma subvar. glabrum, the name of a subvariety of the species Astrophytum myriostigma (bishop's hat cactus).