Flower of the Primrose Willowherb (Ludwigia octovalvis) showing petals and sepals

A sepal is one member or part of the calyx of a flower. It is the outer part of the perianth, which comprises the sterile parts of a flower, consisting of inner and outer tepals that are usually differentiated into petals and sepals. The term tepal is usually applied when the petals and sepals are not differentiated. However, in a "typical" flower the sepals are green and lie under the more conspicuous petals. When the flower is in bud, they enclose and protect the more delicate parts within.

The number of sepals in a flower is indicative of the plant's classification: dicots having typically four or five sepals and monocots having three, or some multiple of three, sepals.

There exists considerable variation in form of the sepals among the flowering plants. Often the sepals are much reduced, appearing somewhat awn-like, or as scales, teeth, or ridges. Examples of flowers with much reduced perianths are found among the grasses. In some flowers, the sepals are fused towards the base, forming a calyx tube. This floral tube can include the petals and the attachment point of the stamens.