A commissioner (commonly abbreviated as Comm'r) is, in principle, a member of a commission or an individual who has been given a commission (official charge or authority to do something).

In practice, the title of commissioner has evolved to include a variety of senior officials, often sitting on a specific commission. In particular, the commissioner frequently refers to senior police or government officials. A high commissioner is equivalent to an ambassador, originally between the United Kingdom and the Dominions and now between all Commonwealth states, whether Commonwealth realms, republics or countries having a monarch other than that of the realms. The title is sometimes given to senior officials in the private sector; for instance, many North American sports leagues.

There is some confusion between commissioners and commissaries because other European languages use the same word for both. Therefore titles such as commissaire in French, Kommissar in German and comisario in Spanish or commissario in Italian, can mean either commissioner or commissary in English, depending on the context.

A commissioner within a modern state generally holds his or her office by a commission from the head of state or a council of elected representatives (or appointed by non-elected officials in the case of dictatorships).

Commissioners are the formal heads of the territories in Canada (those areas under the formal jurisdiction of the federal Crown-in-Council without separate constitutional status of a province). Unlike the governor general or a lieutenant governor, commissioners are not viceregal representatives of the Canadian monarch; rather, they are delegates of the federal Crown-in-Council and, under federal statutes governing the territories, act following written instructions from Cabinet or the minister responsible (currently the minister of northern affairs). While commissioners formerly had a direct day-to-day role in administration and government and chaired the territory's executive council, today they are under instruction to act more like provincial lieutenant governors, as territorial assemblies have taken on more responsibility. Commissioners thus perform ceremonial duties similar to those of the monarch and viceroys, including reading the speech from the throne at the opening of the territorial legislature and presenting commendations to Canadian Forces members for long-term or outstanding service to the office. Possible candidates for the position are selected by the Advisory Committee on Vice-Regal Appointments. The official appointment is made by the Governor General-in-Council (the federal government).

Senior public servants, commissioners and other high-ranking bureaucrats referred to collectively as mandarins.

In the local government system of the Isle of Man, a commissioner is an elected representative equivalent to a councillor. All town, village, district and parish local government bodies consist of commissioners, except for Douglas, which has a council and councillors.

Malawi's position of district commissioner refers to the person that is appointed by the president of Malawi to oversee the administration of any of its 28 districts. One commissioner is appointed per district. The position was created during the British colonial era, sustained during the Kamuzu Banda era and continues as a prominent position in democratic era in Malawi.