The 127mm Mk 45 is a naval gun turret of US origin. It was developed in the 1960's by United Defense as a lighter alternative to the earlier Mk 42 turret. The Mk 45 is the smallest 127mm gun turret in the world and can be considered a direct competitor to the Italian 127mm Compatto. The Mk 45 is a lighter and easier to install design while the Compatto has a higher rate of fire and has more ammunition ready to fire. Both guns use the same US standard 127mm ammunition.

The Mk 45 is a single gun turret which is armed with the 127mm Mk 19 gun which was derived from the earlier Mk 18 that was used in the older Mk 42 turret. The Mk 45 is an unmanned turret with an automatic loader and a 20 round magazine below deck. Additional rounds are stored elsewhere in the ship and fed into the magazine using a feed chute. The gun is controlled using consoles below deck or in the command center. The latest development are a longer barrel and extended range guided munition (ERGM), the latter program was cancelled while the new barrel is in production.

The Mk 45 fires 127mm shells for use against shore targets, naval vessels and aircraft. The gun has a rate of fire of 16 to 20 rounds per minute. The maximum range is 23 km versus surface targets and the anti-aircraft range is quoted as 15 km. The latest Mk 45 design with longer barrel has a longer range and higher rate of fire. The ERGM round has a range of 117 km but was never fielded. Depending on the ship design the total ammunition load ranges between 475 and 680 rounds.

The Mk 45 is used on a wide range of modern US destroyers and cruisers. The Arleigh Burke class destroyers and Ticonderoga class cruisers are the main users of the Mk 45. Due to its ease of installation it was sold to various US allies for use on frigates and destroyers. The Mk 45 Mod 4 remains in production and has been exported as well.