libavcodec is a free software/open source LGPL-licensed library of codecs for encoding and decoding video and audio data. Same name but incompatible libraries are provided from both FFmpeg project and Libav project.

libavcodec is an integral part of many open-source multimedia applications and frameworks. The popular MPlayer, xine and VLC media players use it as their main, built-in decoding engine that enables playback of many audio and video formats on all supported platforms. It is also used by the ffdshow tryouts decoder as its primary decoding library. libavcodec is also used in video editing and transcoding applications like Avidemux, MEncoder or Kdenlive for both decoding and encoding.

libavcodec is particular in that it contains decoder and sometimes encoder implementations of several proprietary formats, including ones for which no public specification has been released. This reverse engineering effort is thus a significant part of libavcodec development. Having such codecs available within the standard libavcodec framework gives a number of benefits over using the original codecs, most notably increased portability, and in some cases also better performance, since libavcodec contains a standard library of highly optimized implementations of common building blocks, such as DCT and color space conversion. However, even though libavcodec strives for decoding that is bit-exact to the official implementation, bugs and missing features in such reimplementations can sometimes introduce compatibility problems playing back certain files.