有損數據壓縮又稱破壞型壓縮，即将次要的信息数据压缩掉，犧牲一些质量來减少数据量，使压缩比提高。它和无损数据压缩相对。
有两种基本的有损压缩机制：
有些系统中同时使用这两种技术，变换编解码用于压缩预测步骤产生的误差信号。
有损方法的一个优点就是在有些情况下能够获得比任何已知无损方法小得多的文件大小，同时又能满足系统的需要。
有损方法经常用于压缩声音、图像以及视频。有损视频编解码几乎总能达到比音频或者静态图像好得多的压缩率（压缩率是压缩文件与未压缩文件的比值）。音频能够在没有察觉的质量下降情况下实现 10:1 的压缩比，视频能够在稍微观察质量下降的情况下实现如 300:1 这样非常大的压缩比。有损静态图像压缩经常如音频那样能够得到原始大小的 1/10，但是质量下降更加明显，尤其是在仔细观察的时候。
When a user acquires a lossily-compressed file, (for example, to reduce download-time) the retrieved file can be quite different from the original at the bit level while being indistinguishable to the human ear or eye for most practical purposes.当用户得到有损压缩文件的时候，
Many methods focus on the idiosyncrasies of the human anatomy, taking into account, for example, that the human eye can see only certain frequencies of light. Thepsychoacoustic model describes how sound can be highly compressed without degrading the perceived quality of the sound. Flaws caused by lossy compression that are noticeable to the human eye or ear are known as compression artifacts.
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The above images show the use of lossy compression to reduce the file size of the image. The image is an excerpt of the image of Lenna, a de facto industry-standard test image.
Even though the third image has high distortion, the face is still recognizable. Good lossy compression algorithms are able to throw away "less important" information and still retain the "essential" information.
从技术的角度来讲，去除文字元音字母也可以看作是有损数据压缩的一种方法，只有辅音的情况下根据上下文通常也仍然可以阅读。研究人员也曾经半开玩笑地用字典中的短单词替换长单词或者使用生成文本技术（:en:generative text techniques）[1]进行文本压缩，尽管这种方法有时是属于相关的有损数据转换（en:Lossy data conversion）领域。