直角三角形的斜边是三角形的最长边，即直角的对边。这个词源于希腊语 hypo- （“在...之下”）和 teinein （“拉伸”）。
直角三角形斜边的长度可以使用勾股定理求得。斜边、邻边和对边的长度出现在三角函数的定义中，尤其通过它们的助记符 SOHCAHTOA。
在吉尔伯特和沙利文的轻歌剧《彭赞斯的海盗》中，斯坦利少将以其关于斜边的知识给海盗们留下了深刻印象，在《少将之歌》中唱道：“I am the very model of a modern Major-General, I've information vegetable, animal, and mineral, I know the kings of England, and I quote the fights historical, From Marathon to Waterloo, in order categorical; I'm very well acquainted too with matters mathematical, I understand equations, both the simple and quadratical, About binomial theorem I'm teeming with a lot o' news-- With many cheerful facts about the square of the hypotenuse."
