Org Mode (also: org-mode; /ˈɔːrɡ moʊd/) is a mode for document editing, formatting, and organizing within the free software text editor GNU Emacs and its derivatives, designed for notes, planning, and authoring. The name is used to encompass plain text files ("org files") that include simple marks to indicate levels of a hierarchy (such as the outline of an essay, a topic list with subtopics, nested computer code, etc.), and an editor with functions that can read the markup and manipulate hierarchy elements (expand/hide elements, move blocks of elements, check off to-do list items, etc.).

Org Mode was created by Carsten Dominik in 2003, originally to organize his own life and work, and since the first release numerous other users and developers have contributed to this free software package. Emacs has included Org Mode as a major mode by default since 2006. Bastien Guerry is the maintainer since 2010, in cooperation with an active development community. Since its success in Emacs, some other systems now provide functions to work with org files.

Almost orthogonally, Org Mode has functionalities aimed at executing code in various external languages; these functionalities form org-babel.

The Org Mode home page explains that "at its core, Org Mode is a simple outliner for note-taking and list management". The Org system author Carsten Dominik explains that "Org Mode does outlining, note-taking, hyperlinks, spreadsheets, TODO lists, project planning, GTD, HTML and LaTeX authoring, all with plain text files in Emacs."

The Org system is based on plain text files with a simple markup, which makes the files very portable. The Linux Information Project explains that "Plain text is supported by nearly every application program on every operating system".

The system includes a lightweight markup language for plain text files (similar in function to Markdown, reStructuredText, Textile, etc., with a different implementation), allowing lines or sections of plain text to be hierarchically divided, tagged, linked, and so on.

This section gives some sample uses for the hierarchical display and editing of plain text.

An org-mode document can also be exported to various formats (including HTML, LaTeX, OpenDocument or plain text), these formats being used to render the structural outline in an appropriate fashion (including cross-references if needed). It can also use formatting markup (including LaTeX for mathematics), with facilities similar to those present in Markdown or LaTeX, thus offering an alternative to these tools.