Colonel (abbreviated as Col., Col or COL) is a senior military officer rank used in many countries. It is also used in some police forces and paramilitary organizations.

In the 17th, 18th and 19th centuries, a colonel was typically in charge of a regiment in an army. Modern usage varies greatly, and in some cases, the term is used as an honorific title that may have no direct relationship to military service. The rank of colonel is typically above the rank of lieutenant colonel. The rank above colonel is typically called brigadier, brigade general or brigadier general. In some smaller military forces, such as those of Monaco or the Vatican, colonel is the highest rank.

Equivalent naval ranks may be called captain or ship-of-the-line captain. In the Commonwealth's air force ranking system, the equivalent rank is group captain.