离散余弦变换（英语：DCT for Discrete Cosine Transform）是与傅里叶变换相关的一种变换，类似于离散傅里叶变换，但是只使用实数。离散余弦变换相当于一个长度大概是它两倍的离散傅里叶变换，这个离散傅里叶变换是对一个实偶函数进行的（因为一个实偶函数的傅里叶变换仍然是一个实偶函数），在有些变形里面需要将输入或者输出的位置移动半个单位（DCT有8种标准类型，其中4种是常见的）。

最常用的一种离散余弦变换的类型是下面给出的第二种类型，通常我们所说的离散余弦变换指的就是这种。它的逆，也就是下面给出的第三种类型，通常相应的被称为"反离散余弦变换"，"逆离散余弦变换"或者"IDCT"。

A discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequencies. DCTs are important to numerous applications in science and engineering, from lossy compression of audio (e.g. MP3) and images (e.g. JPEG) (where small high-frequency components can be discarded), to spectral methods for the numerical solution of partial differential equations. The use of cosine rather than sine functions is critical for compression, since it turns out (as described below) that fewer cosine functions are needed to approximate a typical signal, whereas for differential equations the cosines express a particular choice of boundary conditions.