A constable is a person holding a particular office, most commonly in law enforcement. The office of constable can vary significantly in different jurisdictions. Constable is commonly the rank of an officer within a police service. Other people may be granted powers of a constable without holding this title.

Etymologically, the word constable is a loan from Old French conestable (Modern French connétable), itself from Late Latin comes stabuli (attendant to the stables, literally 'count of the stable'), and originated from the Roman Empire; originally, the constable was the officer responsible for keeping the horses of a lord or monarch.

The title was imported to the monarchies of medieval Europe, and in many countries developed into a high military rank and great officer of state (e.g. the Constable of France, in French Connétable de France, who was the commander-in-chief of all royal armed forces (second to the king) until Prime Minister Cardinal Richelieu abolished the charge in 1627).

Most constables in modern jurisdictions are law enforcement officers. In the United Kingdom, the Commonwealth of Nations and some Continental European countries, a constable is the lowest rank of police officer (it is also, when preceded by the term sworn, used to describe any police officer with arrest and other powers), while in the United States a constable is generally an elected peace officer with lesser jurisdiction than a sheriff; however, in the Channel Islands a constable is an elected office-holder at the parish level.

Historically, a constable could also refer to a castellan, the officer charged with the defence of a castle. Even today, there is a Constable of the Tower of London.

An equivalent position is that of marshal, which is from Old French mareschal (Modern French maréchal), itself from Old Frankish *marskalk, attested by Medieval Latin mariscalcus from a Proto-Germanic *maraχskalkaz (cf. Old High German marahschalh), a compound of *maraz "horse" (cf. English mare) and *skalkaz "servant" (cf. Old English sċealc "servant, retainer, member of a crew") and originally meant "stable keeper, horse tender, groom".

In Australia, as in the United Kingdom, constable is the lowest rank in most police services. It is often categorised into the following (from lowest to highest): probationary constable; constable; constable first class; senior constable; leading senior constable. These variations depend on the individual state or territory police force in question.

Senior constable generally refers to a police officer of the rank above constable, and is denoted by way of two chevrons or stripes.