存在主义（英语：Existentialism），是一个哲学的非理性主义思潮，它认为人存在的意义是无法经由理性思考而得到答案的，以强调个人、独立自主和主观经验。尼采和祁克果可被看作其先驱。在20世纪中它流传非常广泛，其哲学思想还延续到了60年代兴起的人本主义。雅斯贝尔斯和海德格尔、保罗·萨特和作家加缪是其代表人物。

其最突出的命题是：世界没有终极的目标；人们发现自己处于一个隐隐约约而有敌意的世界中；人们选择而且无法避免选择他们的品格、目标和观点；不选择就是一种选择，即是选择了「不选择」；世界和我们的处境的真相最清楚地反映在茫然的心理不安或恐惧的瞬间。

Existentialism (/ɛɡzɪˈstɛnʃəlɪzəm/) is a term applied to the work of certain late-19th- and 20th-century European philosophers who, despite profound doctrinal differences, shared the belief that philosophical thinking begins with the human subject—not merely the thinking subject, but the acting, feeling, living human individual. While the supreme value of existentialist thought is commonly acknowledged to be freedom, its primary virtue is authenticity. In the view of the existentialist, the individual's starting point is characterized by what has been called "the existential attitude", or a sense of disorientation and confusion in the face of an apparently meaningless or absurd world. Many existentialists have also regarded traditional systematic or academic philosophies, in both style and content, as too abstract and remote from concrete human experience.