A troop is a military unit, originally a small force of cavalry, subordinate to a squadron and headed by the troop leader. A cavalry soldier of private rank is called a trooper (abbreviated Tpr., not to be confused with trouper).

A related sense of the term troop refers to soldiers collectively, as in the troops; see Troop (disambiguation).

Troops in various forces

Today, a troop is defined differently in different armed forces.

In the Australian army a troop is the equivalent of a platoon sized element in units of certain corps, those being:

• Royal Australian Armoured Corps

• Royal Australian Engineers

• Royal Australian Corps of Signals

• Australian Army Aviation

• Royal Australian Corps of Transport

• Royal Australian Survey Corps (now disbanded)

In addition the Special Air Service Regiment, which is part of the Royal Australian Infantry Corps, also uses the term troop to refer to its platoon size elements. However SASR is the only Royal Australian Infantry unit to use the troop designation. SASR troops are also unusual as they are commanded by a captain - most troop/platoon sized elements are commanded by a Lieutenant.
In all cases the organisation which use troop to refer to their platoon size element refer to the company sized element as a squadron and the battalion sized element as a regiment.
Privates in the RAAC and SASR are also hold the rank "Trooper" instead of private, this is not the case for any other Corps/Units whose platoon sized elements are called Troops.
In the British Army the definition of a troop varies by corps.

• Household Cavalry and Royal Armoured Corps: Three or four armoured fighting vehicles commanded by a subaltern, i.e. effectively the same level element as an infantry platoon. A unit of two to four guns or launchers, or an equivalent headquarters unit. In the Royal Horse Artillery, a troop used to be the equivalent to a battery in other artillery units.

• Royal Engineers, Royal Corps of Signals, Royal Logistic Corps, Special Air Service and Honourable Artillery Company: A unit equivalent in size to a platoon in other corps, divided into sections or Patrols.

Other army corps do not use the term.

In the Royal Marines, a troop is the equivalent to an army platoon.

In the Canadian Army, a Troop is the equivalent of a platoon within the Armoured, Engineer, and Signals branches. Two to four Troops are comprise the main elements of a squadron.

In the United States Army, in the cavalry branch, a troop is the equivalent unit to the infantry company, commanded by a captain and consisting of 3 or 4 platoons, and subordinate to a squadron (battalion).

Troops in civilian organizations

In the United States, State Police forces are often regionally divided into Troops. This usage came about from these organizations modeling themselves off the US Army, and especially the older cavalry units. For this same reason the State Police and Highway Patrol personnel of most states are known as "Trooper" rather than "Officer".

In Scouting, a troop is a unit made up of Scouts or Guides from the same locality under a leader.