OpenOffice.org (OOo), commonly known as OpenOffice, is a discontinued open-source office suite. It was an open-sourced version of the earlier StarOffice, which Sun Microsystems acquired in 1999 for internal use. Sun open-sourced the OpenOffice suite in July 2000 as a competitor to Microsoft Office, releasing version 1.0 on 1 May 2002.OpenOffice included a word processor (Writer), a spreadsheet (Calc), a presentation application (Impress), a drawing application (Draw), a formula editor (Math), and a database management application (Base). Its default file format was the OpenDocument Format (ODF), an ISO/IEC standard, which originated with OpenOffice.org. It could also read a wide variety of other file formats, with particular attention to those from Microsoft Office. OpenOffice.org was primarily developed for Linux, Microsoft Windows and Solaris, and later for OS X, with ports to other operating systems. It was distributed under the GNU Lesser General Public License version 3 (LGPL); early versions were also available under the Sun Industry Standards Source License (SISSL).

In 2011, Oracle Corporation, the then-owner of Sun, announced that it would no longer offer a commercial version of the suite and donated the project to the Apache Foundation. Apache renamed the software Apache OpenOffice. Other active successor projects include LibreOffice (the most actively developed) and NeoOffice (commercial, and available only for macOS).