The 4.5 inch Mark 8 is a British naval gun system which currently equips the Royal Navy's frigates and some British destroyers and frigates sold to other countries.

The 4.5 inch gun has been the standard medium-gun calibre of the Royal Navy for use against surface, aircraft and shore targets since 1938. The current 55-calibre Mark 8 gun replaced the World War II era 45-calibre QF 4.5 inch Mk I - V naval guns. Like all British 4.5 inch naval guns, it actually has a calibre of 4.45 inches (113 mm).

A completely new type of 4.5 inch gun with a longer 55-calibre barrel, it was designed in the 1960s for the Royal Navy's new classes of frigates and destroyers. The new weapon, built by Vickers^{[citation needed]}, was developed by the Royal Armament Research and Development Establishment using the Ordnance, QF 105 mm L13 of the Abbot self-propelled gun as a starting point (it used electrical primers). The outer shell of the gunhouse is built from glass-reinforced plastic (GRP).

The new weapon emphasised reliability over a high rate of fire, allowing a switch to a lighter, single barrel mounting and ammunition of a one-piece design.