脊索（英语：notochord)，是条不仅长而韧的而且在背部中起支持体轴作用的棒状结构。脊索位于消化道背面和背神经管腹面之间，来源于脊索动物的胚胎的原肠背壁。 有的品种，例如文昌鱼，圆口类。终生都靠脊索支撑身体。

In animal anatomy, the notochord is a flexible rod made out of a material similar to cartilage. If a species has a notochord, it is, by definition, a chordate. The notochord lies along the anteroposterior ("head to tail") axis, is usually closer to the dorsal than the ventral surface of the animal, and is composed of cells derived from the mesoderm. The notochord has been observed to have many functions including developmental functions. The most commonly cited functions are as a site of muscle attachment, vertebral precursor, and as a midline tissue that provides signals to the surrounding tissue during development.