Fast Infoset (or FI) is an international standard that specifies a binary encoding format for the XML Information Set (XML Infoset) as an alternative to the XML document format. It aims to provide more efficient serialization than the text-based XML format.

One can think of FI as gzip for XML, though FI aims to optimize both document size and processing performance, whereas gzip optimizes only the size. While the original formatting is lost, no information is lost in the conversion from XML to FI and back to XML.

The Fast Infoset specification is defined by both the ITU-T and the ISO standards bodies. FI is officially named ITU-T Rec. X.891 and ISO/IEC 24824-1 (Fast Infoset), respectively. However, it is commonly referred to by the name Fast Infoset. The standard was published by ITU-T on May 14, 2005, and by ISO on May 4, 2007.