An ex officio member is a member of a body (notably a board, committee, 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.