The Mark 54 Lightweight Torpedo (formerly known as Lightweight Hybrid Torpedo, or LHT) is a standard 12.75-inch (324 mm) anti-submarine warfare (ASW) torpedo used by the United States Navy.

The Mark 54 was co-developed by Raytheon Integrated Defense Systems and the U.S. Navy under the U.S. Navy's Lightweight Hybrid Torpedo program in response to perceived problems with the extant Mark 50 and Mark 46 torpedoes. The Mk 50, having been developed to counter very high performance nuclear submarines such as the Soviet Alfa class, was seen as too expensive to use against relatively slow conventional submarines. The older Mk 46, designed for open-ocean use, performed poorly in the littoral areas, where the Navy envisioned itself likely to operate in the future.

The Mk 54 was created by combining the homing portion of the Mk 50 and the warhead and propulsion sections of the Mk 46, improved for better performance in shallow water, and with the addition of commercial off-the-shelf (COTS) technology to further reduce costs. It shares much of the software and computer hardware of the Mk 48 ADCAP heavy torpedo, based around a custom PowerPC 603e microprocessor.

Developmental testing began in July 1999, and a successful critical design review was completed in November 1999.

In April 2003, Raytheon was awarded a sole source contract for the production of the Mk 54. Full rate production began in October 2004. In March 2010 the Fifth Fleet requested improvements in the Mk 54's performance against diesel-electric submarines via an Urgent Operational Need Statement (UONS). This led to a software Block Upgrade (BUG) program which began testing in August 2011 and which continues, having been criticised by the Director, Operational Test and Evaluation (DOT&E) for using unrealistic proxies for threat submarines.^[3]

The Mk 54 can be fired from surface ships via the Mark 32 surface vessel torpedo tubes or the vertical launch anti-submarine rocket (ASROC) systems, and also from most ASW aircraft, although they are slightly different lengths and weights. The P-8 Poseidon uses the High-Altitude Anti-Submarine Warfare Weapons Capability (HAAWC) GPS-guided parachute kit to drop torpedoes from high altitude.

The FY14 DOT&E report assessed the Mk 54 (BUG) torpedo as not operationally effective in its intended role: "During operationally challenging and realistic scenarios, the Mk 54 (BUG) demonstrated below threshold performance and exhibited many of the same failure mechanisms observed during the FY 2004 initial operational testing". Shortfalls were also identified with the employing platforms’ tactics and tactical documentation, and interoperability problems with some platform fire control systems.^[4]

• APR-3E torpedo - Russian equivalent
• A244-S - Italian equivalent
• MU90 Impact - French/Italian equivalent
• Sting Ray (torpedo) - British equivalent
• TAL Shyena - Indian equivalent
• Yu-7 torpedo - Chinese equivalent
• K745 Chung Sang Eo - South Korean equivalent
• Type 97 light weight torpedo (G-RX4) - Japanese equivalent