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.