The Phalanx CIWS (SEE-wiz) is an automated gun-based close-in weapon system to defend military watercraft automatically against incoming threats such as aircraft, missiles, and small boats. It was designed and manufactured by the General Dynamics Corporation, Pomona Division, later a part of Raytheon. Consisting of a radar-guided 20 mm (0.8 in) Vulcan cannon mounted on a swiveling base, the Phalanx has been used by the United States Navy and the naval forces of 15 other countries. The U.S. Navy deploys it on every class of surface combat ship, except the Zumwalt-class destroyer and San Antonio-class amphibious transport dock. Other users include the British Royal Navy, the Royal Australian Navy, the Royal New Zealand Navy, the Royal Canadian Navy, and the U.S. Coast Guard.

A land variant, the LPWS (Land Phalanx Weapon System), part of the Counter Rocket, Artillery, and Mortar (C-RAM) system, was developed. It was deployed to counter rocket, artillery and mortar attacks during the 2021 US withdrawal from Afghanistan. The U.S. Navy also fields the SeaRAM system, which pairs the RIM-116 Rolling Airframe Missile with sensors based on the Phalanx.

The Phalanx Close-In Weapon System (CIWS) was developed as the last line of automated weapons defense (terminal defense or point defense) against all incoming threats, including antiship missiles (AShMs or ASMs), aircraft including high-g and maneuvering sea-skimmers, and small boats.

The first prototype system was offered to the U.S. Navy for evaluation on the destroyer leader USS King in 1973 and it was determined that further work was required to improve performance and reliability. Subsequently, the Phalanx Operational Suitability Model successfully completed its Operational Test and Evaluation (OT&E) on board the destroyer USS Bigelow in 1977. The model exceeded operational maintenance, reliability, and availability specifications. Another evaluation successfully followed, and the weapon system was approved for production in 1978. Phalanx production started with orders for 23 USN and 14 foreign military systems. The first ship fully fitted out was the aircraft carrier USS Coral Sea in 1980. The Navy began placing CIWS systems on non-combatant vessels in 1984.

The basis of the system is the 20 mm M61 Vulcan rotary cannon, used by the United States military on various tactical aircraft since 1959, linked to a K_u band fire control radar system for acquiring and tracking targets. This proven system was combined with a purpose-made mounting, capable of fast elevation and traverse speeds, to track incoming targets. An entirely self-contained unit, the mounting houses the gun, an automated fire-control system and all other major components, enabling it to automatically search for, detect, track, engage, and confirm kills using its computer-controlled radar system. Owing to this self-contained nature, Phalanx is ideal for support ships, which lack integrated targeting systems and generally have limited sensors. The entire unit has a mass between 12,400 to 13,500 lb (5,600 to 6,100 kg).^{[citation needed]}

Because of their distinctive barrel-shaped radome and their automated operation, Phalanx CIWS units are sometimes nicknamed "R2-D2" after the droid from the Star Wars films.

The Phalanx system has been developed through several configurations. The basic (original) is the Block 0, equipped with first-generation, solid-state electronics and with marginal capability against surface targets. The Block 1 (1988) upgrade improved radar, ammunition, computing power, rate of fire, and an increase in maximum engagement elevation to +70 degrees. These improvements were intended to increase the system's capability against emerging Russian supersonic anti-ship missiles. Block 1A introduced a new computer system to counter more maneuverable targets. The Block 1B PSuM (Phalanx Surface Mode, 1999) adds a forward-looking infrared (FLIR) sensor to make the weapon effective against surface targets. This addition was developed to provide ship defense against small vessel threats and other "floaters" in littoral waters and to improve the weapon's performance against slower low-flying aircraft. The FLIR's capability is also of use against low-observability missiles and can be linked with the RIM-116 Rolling Airframe Missile (RAM) system to increase RAM engagement range and accuracy. The Block 1B also allows for an operator to visually identify and target threats.

Since the end of FY 2015, the US Navy has upgraded all Phalanx systems to the Block 1B variant. In addition to the FLIR sensor, the Block 1B incorporates an automatic acquisition video tracker, optimized gun barrels (OGB), and Enhanced Lethality Cartridges (ELC) for additional capabilities against asymmetric threats such as small maneuvering surface craft, slow-flying fixed and rotary-winged aircraft, and unmanned aerial vehicles. The FLIR sensor improves performance against anti-ship cruise missiles, while the OGB and ELC provide tighter dispersion and increased "first-hit" range; the Mk 244 ELC is specifically designed to penetrate anti-ship missiles with a 48 percent heavier tungsten penetrator round and an aluminum nose piece. Another system upgrade is the Phalanx 1B Baseline 2 radar to improve detection performance, increase reliability, and reduce maintenance. It also has a surface mode to track, detect, and destroy threats closer to the water's surface, increasing the ability to defend against fast-attack boats and low-flying missiles. As of 2019, the Baseline 2 radar upgrade has been installed on all U.S. Navy Phalanx system-equipped vessels. The Block 1B is also used by other navies, such as Canada, Portugal, Japan, Egypt, Bahrain, and the UK.