Office Open XML (also informally known as OOXML or OpenXML) is a zipped, XML-based developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. The Office Open XML specification was initially standardised by Ecma (as ECMA-376) and later by ISO and IEC (as ISO/IEC 29500).

Starting with Microsoft Office 2007, the Office Open XML file formats have become the default target file format of Microsoft Office. Microsoft Office 2010 provides read support for ECMA-376, read/write support for ISO/IEC 29500 Transitional, and read support for ISO/IEC 29500 Strict. Microsoft Office 2013 additionally supports both reading and writing of ISO/IEC 29500 Strict.