The Greater Underwater Propulsion Power Program (GUPPY) was initiated by the United States Navy after World War II to improve the submerged speed, maneuverability, and endurance of its submarines. (The "Y" in the acronym was added for pronounceability.)

The navy began the program by testing and reverse engineering two German Type XXI U-boats—U-2513 and U-3008—obtained as war reparation. That analysis led to four goals—increasing the submarines' battery capacity, streamlining the boats' structures, adding snorkels, and improving fire control systems. The navy immediately focused on designing a new class of submarine, but the Bureau of Ships believed the fleet of existing Gato, Balao, and Tench-class submarines could be modified to incorporate the desired improvements. In June 1946, the Chief of Naval Operations approved the GUPPY project. The initial two-boat test program, implemented by the Portsmouth Naval Shipyard, eventually grew into several successive conversion programs. Those upgrades proceeded in seven variants, in the following order: GUPPY I, GUPPY II, GUPPY IA, Fleet Snorkel, GUPPY IIA, GUPPY IB, and GUPPY III. Some boats that went through an early phase were then upgraded further in a later phase. Most GUPPY phases were assigned a corresponding Ship Characteristics Board / SCB project number.

A similar program for the Royal Navy involved modifications to 24 wartime and post-war British T- and A-class submarines, which were provided with streamlined hulls, fin-type conning towers, and increased underwater performance during 1948-60.