紫外线（Ultraviolet或简称UV）是波长比可见光短，但比X射线长的电磁辐射，波长范围在10纳米（nm）至400纳米，能量从3电子伏特（eV）至124电子伏特之间。它的名称是因为在光谱中电磁波频率比肉眼可见的紫色还要高而得名，又俗称紫外光。

虽然人的肉眼看不见紫外线，但紫外线却会造成晒伤的影响。紫外线还有其他的效应，对人类的健康既有益处也有害处。

紫外线出现在阳光中，并且也能在电弧和专门的灯中生成，例如水银灯，日晒灯和黑光灯。尽管紫外线的能量不足以将原子电离，它可以造成化学反应，并导致许多物质发光或产生萤光。大多数紫外光被归类为非电离辐射。能量较高的紫外线，大约在150纳米（真空紫外线）是电离的，但这种类型的紫外线不具穿透力，会被空气阻挡住。

Ultraviolet (UV) light is an electromagnetic radiation with a wavelength from 10 nm (30 PHz) to 380 nm (750 THz), shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, and also produced by electric arcs and specialized lights such as mercury-vapor lamps, tanning lamps, and black lights. Although lacking the energy to ionize atoms, long-wavelength ultraviolet radiation can cause chemical reactions, and causes many substances to glow or fluoresce. Consequently, biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.