抗原表位（英语：antigenic epitope），简称“表位”，也称为“抗原决定簇”（antigenic determinant），是指抗原表面上决定抗原特异性的化学基团。抗原表位可被免疫系统（尤其是抗体、B细胞或者T细胞）所识别。抗体中能识别抗原表位的区域叫做“互补位”或“抗体决定簇”。尽管通常抗原表位是指外来蛋白质等物质的其中一部分，但只要能被自身免疫系统所识别的表位，也被归为抗原表位。

蛋白质抗原的表位根据它们的结构以及与互补位的交互作用，被分为构象表位和线性表位这两种类型。其中构象表位有抗原氨基酸串行中的不连续部分组成，因此互补位和抗原表位的交互作用是基于表面的三位特征和形状，或者是抗原的三级结构。大部分的抗原表位都属于构象表位。与此相反，线性表位是由一段连续的抗原氨基酸串行构成，与抗原的交互作用的基础是其一级结构。

An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. In other words, the epitope is the specific piece of the antigen to which an antibody binds. The part of an antibody that binds to the epitope is called a paratope. Although epitopes are usually non-self proteins, sequences derived from the host that can be recognized (as in the case of autoimmune diseases) are also epitopes.