在数学领域中，克莱因瓶（德语：Kleinsche Flasche）是指一种无定向性的平面，比如二维平面，就没有“内部”和“外部”之分。克莱因瓶最初的概念提出是由德国数学家菲利克斯·克莱因提出的。克莱因瓶和莫比乌斯带非常相像。

要想像克莱因瓶的结构，可先试想一个底部镂空的红酒瓶。现在延长其颈部，向外扭曲后伸进瓶子的内部，再与底部的洞相连接。

和我们平时用来喝水的杯子不一样，这个物体没有“边”，它的表面不会终结。它也不类似于气球，一只苍蝇可以从瓶子的内部直接飞到外部而不用穿过表面（所以说它没有内外部之分）。

In mathematics, the Klein bottle /ˈklaɪn/ is an example of a non-orientable surface; it is a two-dimensional manifold against which a system for determining a normal vector cannot be consistently defined. Informally, it is a one-sided surface which, if traveled upon, could be followed back to the point of origin while flipping the traveler upside down. Other related non-orientable objects include the Möbius strip and the real projective plane. Whereas a Möbius strip is a surface with boundary, a Klein bottle has no boundary (for comparison, a sphere is an orientable surface with no boundary).