衔尾蛇（希腊语：οὐροβόρος，亦作咬尾蛇，音译奥罗波若蛇），是一个自古代流传至今的符号，大致形象为一条蛇（或龙）正在吞食自己的尾巴，结果形成出一个圆环（有时亦会展示成扭纹形，即阿拉伯数字「8」的形状），其名字涵义为「自我吞食者」。这个符号一直都有很多不同的象征意义，而当中最为人接受的是「无限大」、「循环」等意义。另外，衔尾蛇亦是宗教及神话中的常见符号，在炼金术中更是重要的徽记。近代，有些心理学家（如卡尔·荣格）认为，衔尾蛇其实反映了人类心理的原型。

The ouroboros or uroboros (/ˌjʊərəˈbɒrəs, ˌjʊəroʊ-/, from the Greek οὐροβόρος ὄφις tail-devouring snake) is an ancient symbol depicting a serpent or dragon eating its own tail.

The ouroboros often symbolizes self-reflexivity, introspection, or cyclicality, especially in the sense of something constantly re-creating itself, the eternal return, and other things such as the phoenix which operate in cycles that begin anew as soon as they end. It can also represent the idea of primordial unity related to something existing in or persisting from the beginning with such force or qualities it cannot be extinguished. While first emerging in Ancient Egypt and India, the ouroboros has been important in religious and mythological symbolism, but has also been frequently used in alchemical illustrations, where it symbolizes the circular nature of the alchemist's opus. It is also often associated with Gnosticism, Hermeticism and Hinduism.