Undersecretary (or under secretary) is a title for a person who works for and has a lower rank than a secretary (person in charge). It is used in the executive branch of government, with different meanings in different political systems, and is also used in other organizational settings.

In government, the position may be a junior government minister (e.g. a parliamentary secretary) who assists a secretary of state. In other cases, the position may be a senior government official, frequently a career public servant, who typically acts as a senior administrator. The senior administrator may be considered a second-in-command to a politically appointed cabinet minister or other government official (e.g. in the United States), or they may be considered a head or chief executive of a government department (e.g. a permanent secretary). Some systems of government have both types of position, as in the United Kingdom where the title has been in use since the 17th century.