糖基（英语：glycosyl group）是在移除环状单糖或低聚寡糖的半缩醛羟基之后得到的一价的自由基或取代基结构。 糖基亦可以与如磷酸等的无机酸反应，形成酯，例如葡萄糖1-磷酸。

在纤维素中，糖基以1,4-β-D-葡萄糖单元的形式形成(1,4-beta-D-glucosyl)_n型的长链。另一个例子是6,7-二甲基-8-核糖醇基二氧四氢蝶啶中的核糖醇基。

A glycosyl group is a univalent free radical or substituent structure obtained by removing the hemiacetal hydroxyl group from the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide. Glycosyl also reacts with inorganic acids, such as phosphoric acid, forming an ester such as glucose 1-phosphate.