百日菊（学名：Zinnia elegans），又称百日草，为菊科百日菊属下的一个种。

Zinnia elegans, known as youth-and-age, common zinnia or elegant zinnia, an annual flowering plant of the genus Zinnia, is one of the best known zinnias. It is native to Mexico but grown as an ornamental in many places and naturalized in several places including scattered locations in South and Central America, the West Indies, the United States, Australia, and Italy.