水印是一种容易识别、被夹于纸内，能够透过光线穿过从而显现出各种不同阴影的技术。水印有时也会在反射强光时或者在暗色背景上方时被看到。此种现象是由于纸张厚度或者纸张的密度变化所引起的。生产带有水印的纸张主要有两种方法，压胶辊工艺和圆网印花工艺。

不同水印的可视程度有着非常巨大的区别。有些水印需要了解一些方法才能让其显现出来，另一些却很容易就可观察。各种能够制造出水印效果的简单方法也逐渐被研究出来，如水印液（watermark fluid）甚至可以在不破坏纸张 的前提下显现出水印效果。水印常常起到验证货币、护照、邮票、政府机关文档或者其他纸制文档的真实性的作用。

A watermark is an identifying image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper. Watermarks have been used on postage stamps, currency, and other government documents to discourage counterfeiting. There are two main ways of producing watermarks in paper; the dandy roll process, and the more complex cylinder mould process.