An ex officio member is a member of a body (notably a board, committee, or council) who is part of it by virtue of holding another office. The term ex officio is Latin, meaning literally 'from the office', and the sense intended is 'by right of office'; its use dates back to the Roman Republic.

According to Robert's Rules of Order, the term denotes only how one becomes a member of a body. Accordingly, the rights of an ex officio member are exactly the same as other members unless otherwise stated in regulations or bylaws. It relates to the notion that the position refers to the position the ex officio holds, rather than the individual that holds the position. In some groups, ex officio members may frequently abstain from voting.

Opposite notions are dual mandate, when the same person happens to hold two offices or more, although these offices are not in themselves associated; and personal union, when two states share the same monarch.

Any ex officio membership (for example, of committees, or of the board) is as defined by the nonprofit association's bylaws or other documents of authority. For example, the bylaws quite often provide that the organization's president will be ex officio a member of all committees, except the nominating committee.

The President of the French Republic and the Catholic Bishop of Urgell are by virtue of office (ex officio) appointed Co-Princes of Andorra.

Within the Australian political system, the Liberal–National Coalition is an alliance of centre-right political parties that forms one of the two major groupings in Australian federal politics. The two major partners in the Coalition are the Liberal Party of Australia and the National Party of Australia with each partner being a stand-alone organisation that elects its own leadership. The leader of the Liberals generally serves as the ex officio Leader of the Coalition whereas the leader of the Nationals serves as the ex officio Deputy Leader of the Coalition.

In the above arrangement, when a Coalition is elected to government in Australia or the countries States or territories, convention is that the larger partner, usually the Liberals, hold the office of Prime Minister, Premier or Chief Minister whereas the smaller partner, usually the Nationals, hold office as the Deputy Prime Minister or Deputy Premier. The respective offices are determined based on the size of the respective alliance partner and their independently of one another chosen leader.

The Coalition generally results in the Prime Minister or Premier from the respective jurisdiction being from a metropolitan area (the Liberals) and guarantees a rural or regional Deputy Prime Minister or Premier (from the Nationals). The Leader of the Nationals is usually given the right to choose his or her ministry within government by virtue of being the second-most senior position within the Coalition partnership.