DeCSS is one of the first free computer programs capable of decrypting content on a commercially produced DVD video disc. Before the release of DeCSS, free and open source operating systems (such as BSD and Linux) could not play encrypted video DVDs.

DeCSS's development was done without a license from the DVD Copy Control Association (CCA), the organization responsible for DVD copy protection—namely, the Content Scramble System (CSS) used by commercial DVD publishers. The release of DeCSS resulted in a Norwegian criminal trial and subsequent acquittal of one of the authors of DeCSS. The DVD CCA launched numerous lawsuits in the United States in an effort to stop the distribution of the software.