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SubStation Alpha (or Sub Station Alpha), abbreviated SSA, is a subtitle file format created by Kotus that allows for more advanced subtitles than the conventional SRT and similar formats. To render these subtitles on Microsoft Windows, VSFilter is needed in conjunction with a DirectShow-aware video player.

It is also the name of the popular, now discontinued, tool used to edit the subtitles. See below for more on SSA / ASS subtitle editors.

This subtitle format is frequently used in anime fansubs, either to overlay subtitles onto video while it is being encoded (hardsubbing), or to store subtitle data alongside video data in a Matroska (MKV) container (softsubbing). Hardsubbing is irreversible and does not require VSFilter or other special resources for playback. Softsubbing allows the end user of choose whether subtitles will be displayed, and makes it possible to include multiple subtitle streams in the same video file.