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.

In France the Undersecretary of State (sous-secrétaire d'État) was a governmental role created during the Hundred Days and institutionalized in 1816. It served as an administrative and, later, political assistant to ministers, evolving significantly across the country's political regimes until being replaced by the title of Secretary of State under the Fifth Republic.

In the Holy See, undersecretary is a title given to senior administrative officials working in the dicasteries of the Roman Curia.

In Hong Kong, undersecretary is a position created in 2008 to work closely with bureau secretaries and top civil servants to implement the Chief Executive's policy blueprint and agenda in an executive-led government. The appointment of undersecretaries and political assistants is an extension of the Principal Officials Accountability System into today's Political Appointments System.

In India, undersecretary is an officer level secretariat post in central ministries, tenanted by officers of various civil services, usually promoted officers from the Group 'B' (section officers) of the Central Secretariat Service, or the newly appointed officers of the Indian Administrative Service and the Indian Foreign Service, or other Group 'A' services on deputation in the central government in early stages of their careers with a minimum of 7–8 years of service.

Officers in this capacity further manage desk or section officers within a ministry or central regulatory body. Undersecretaries occupy gazetted civil service posts in India, unlike in some Western democracies, where they are political appointees. An undersecretary will typically report to a deputy secretary, or a director to the Government of India.

In Italy, Undersecretary is a second-in-command to a politically appointed Minister. In particular, he/she is appointed with a President of the Republic Decree, on proposal of the Prime Minister, who previously agreed such proposal with the relevant Minister.