根茎（英语：Rhizome）是变态茎的一种。某些植物的枝干部分，但是并不在地面以上生长，而是在土壤中生长，从形态上看，又似植物的根。但根的作用是吸收土壤中的水和矿物质，而根茎不是，它只是扩展植物的体积。类似的现象还有有块茎。

In botany and dendrology, a rhizome (/ˈraɪzoʊm/, from Ancient Greek: rhízōma "mass of roots", from rhizóō "cause to strike root") is a modified subterranean stem of a plant that is usually found underground, often sending out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks and rootstocks. Rhizomes develop from axillary buds and are diageotropic or grow perpendicular to the force of gravity. The rhizome also retains the ability to allow new shoots to grow upwards.