A patrol boat is a small naval vessel generally designed for coastal defence duties. They may be operated by a nation's navy, coast guard, or even police force and may be intended for marine (blue water) and/or estuarine or river (brown water) environments. They are commonly found engaged in various border protection roles, including anti-smuggling, anti-piracy, fisheries patrols, and immigration law enforcement. They are also often called upon to participate in rescue operations.

Patrol boats are typically around 30m (100 ft) in length and usually carry a single gun as main armament, along with a variety of lighter secondary armament such as machine guns. They are usually diesel-powered, although some modern designs use gas turbines, and speeds are generally in the 25-30 knot range.

Examples in use around the world include the Island class, and the Pacific class .

Similar vessels for exclusively military duties include torpedo boats and missile boats . The U.S. Navy operated the Pegasus class of armed hydrofoils for years, in a patrol boat role.