A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their last years of large-scale manufacture during World War II. Reciprocating steam engines were progressively replaced in marine applications during the 20th century by steam turbines and marine diesel engines.

Scotia was a British passenger liner operated by the Cunard Line that won the Blue Riband in 1863 for the fastest westbound transatlantic voyage. She was the last oceangoing paddle steamer, and as late as 1874 she made Cunard's second fastest voyage. Laid up in 1876, Scotia was converted to a twin-screw cable layer in 1879. She served in her new role for twenty-five years until she was wrecked off of Guam in March 1904.