An FB2 file is a FictionBook 2.0 file saved in an eBook format developed in Russia. It contains XML that describes each element of the book and is designed primarily for fictional literature. FB2 files can be viewed by various eBook readers for Windows, macOS, and Linux.

The FB2 file format specifies the structure of the eBook instead of the appearance. This makes it useful for converting into other formats. The format features simple semantic markup, metadata embedding, unicode, and inline formatting. Since FB2 files can grow large in size, due to embedded images, they are often compressed in .ZIP files in order to transfer them more easily over the Internet. In these cases, the ".fb2" extension may be replaced with the ".zip" or ".fb2.zip" extension.