The Commonwealth secretary-general, formally the secretary-general of the Commonwealth of Nations, is the head of the Commonwealth Secretariat, the central body which has served the Commonwealth of Nations since its establishment in 1965, and responsible for representing the Commonwealth publicly. The Commonwealth secretary-general should not be confused with the head of the Commonwealth.

The position was created, along with the Secretariat itself, after the fourteenth Commonwealth Prime Ministers Conference in London in 1965, issued a memorandum describing the role of the Secretary-General:

Both the Secretary-General and his/her staff should be seen to be the servants of Commonwealth countries collectively. They derive their functions from the authority of Commonwealth Heads of Government; and in the discharge of his/her responsibilities in this connection the Secretary-General should have access to Heads of Government...

The headquarters of the secretary-general, as with the Secretariat generally, is Marlborough House, a former royal residence in London, which was placed at the disposal of the Secretariat by Queen Elizabeth II, who was the head of the Commonwealth. However, as the building cannot house all of the Secretariat's staff in London, additional space is rented elsewhere in London. From this operational base, a large part of the secretary-general's work involves travelling around the Commonwealth keeping in personal contact with those at the heart of the governments of member states.

The secretary-general receives a salary of nearly £160,000 (2015) and a four‑storey mansion, Garden House, in Mayfair as an official residence.

The secretary-general leads the Commonwealth Secretariat, and all Secretariat staff are responsible and answerable to them. They are supported by three deputy secretaries-general, which are elected by the Commonwealth heads of government via the members' high commissioners in London. Currently, the three deputy secretaries-general are Deodat Maharaj, Gary Dunn and Josephine Ojiambo. Until 2014, only two deputy secretaries-general were appointed along with an assistant secretary-general for corporate affairs. The secretary-general may appoint junior staff at their own discretion, provided the Secretariat can afford it, whilst more senior staff may be appointed only from a shortlist of nominations from the heads of government. In practice, the Secretary-General has more power than this; member governments consult the secretary-general on nominations, and the secretary-general has also at times submitted nominations of his own.

Formally, the secretary-general is given the same rank as a high commissioner or ambassador. However, in practice, their rank is considerably higher. At CHOGMs, they are the equal of the heads of government, except with preference deferred to the longest-serving head of government. At other ministerial meetings, they are considered primus inter pares. For the first three years of the job's existence the Foreign Office refused to invite the secretary-general to the Queen's annual diplomatic reception at Buckingham Palace, much to Arnold Smith's irritation, until in 1968 this refusal was over-ridden by Queen Elizabeth II herself.

The secretary-general was originally required to submit annual reports to the heads of government, but this has since been changed to reporting at biennial Commonwealth Heads of Government Meetings (CHOGM). The secretary-general is held responsible by the Commonwealth's Board of Governors in London.