表面张力（Surface tension）是种物理效应，使得液体表面总是获得最小的、光滑的面积，就像是一层弹性的薄膜。原因是液体的表面总是试图达到能量最低的状态。

广义地所有两种不同物态的物质之间界面上的张力被称为表面张力。

表面张力的量纲是单位长度的力和单位面积的能。

在材料科学里，表面张力也称为表面应力和表面自由能。

热力学对表面张力系数的广义定义为：表面张力系数σ是在温度T和压力p不变的情况下吉布士自由能G对面积A的偏导数：

吉布士自由能的单位是能量单位，因此表面张力系数的单位是能量/面积。

Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. Surface tension allows insects (e.g. water striders), usually denser than water, to float and stride on a water surface.

At liquid-air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane. Thus, the surface becomes under tension from the imbalanced forces, which is probably where the term "surface tension" came from. Because of the relatively high attraction of water molecules for each other, water has a higher surface tension (72.8 millinewtons per meter at 20 °C) compared to that of most other liquids. Surface tension is an important factor in the phenomenon of capillarity.