The 180 mm gun S-23 () was a Soviet heavy gun of Cold War era. It was developed in the early 1950s, with the design based on naval guns. Its first public appearance was the 1955 May Day parade in Moscow. For some time, it was believed in the West that the S-23 was actually a 203mm weapon, and as a result it was often referred to as the 203 mm M1955 gun howitzer. However, after an example was captured in the Middle East during the 1970s this misconception was dispelled.