A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships came into practical usage during the early 19th century; however, there were exceptions that came before. Steamships usually use the prefix designations of "PS" for paddle steamer or "SS" for screw steamer (using a propeller or screw). As paddle steamers became less common, "SS" is incorrectly assumed by many to stand for "steamship". Ships powered by internal combustion engines use a prefix such as "MV" for motor vessel, so it is not correct to use "SS" for most modern vessels.

As steamships were less dependent on wind patterns, new trade routes opened up. The steamship has been described as a "major driver of the first wave of trade globalization (1870–1913)" and contributor to "an increase in international trade that was unprecedented in human history".

Steamships were preceded by smaller vessels, called steamboats, conceived in the first half of the 18th century by Denis Papin, with the first working steamboat and paddle steamer, the Pyroscaphe, from 1783. Once the technology of steam was mastered at this level, steam engines were mounted on larger, and eventually, ocean-going vessels. Becoming reliable, and propelled by screw rather than paddlewheels, the technology changed the design of ships for faster, more economic propulsion.

Paddlewheels as the main motive source became standard on these early vessels. It was an effective means of propulsion under ideal conditions but otherwise had serious drawbacks. The paddle-wheel performed best when it operated at a certain depth, however when the depth of the ship changed from added weight it further submerged the paddle wheel causing a substantial decrease in performance.

Within a few decades of the development of the river and canal steamboat, the first steamships began to cross the Atlantic Ocean. The first sea-going steamboat was Richard Wright's first steamboat Experiment, an ex-French lugger; she steamed from Leeds to Yarmouth in July 1813.

The first iron steamship to go to sea was the 116-ton Aaron Manby, built in 1821 by Aaron Manby at the Horseley Ironworks, and became the first iron-built vessel to put to sea when she crossed the English Channel in 1822, arriving in Paris on 22 June. She carried passengers and freight to Paris in 1822 at an average speed of 8 knots (9.2 mph; 15 km/h).

The American ship SS Savannah first crossed the Atlantic Ocean arriving in Liverpool, England, on June 20, 1819, although most of the voyage was actually made under sail. The first ship to make the transatlantic trip substantially under steam power may have been the British-built Dutch-owned Curaçao, a wooden 438-ton vessel built in Dover and powered by two 50 hp engines, which crossed from Hellevoetsluis, near Rotterdam on 26 April 1827 to Paramaribo, Surinam on 24 May, spending 11 days under steam on the way out and more on the return. Another claimant is the Canadian ship SS Royal William in 1833.

The first steamship purpose-built for regularly scheduled trans-Atlantic crossings was the British side-wheel paddle steamer SS Great Western built by Isambard Kingdom Brunel in 1838, which inaugurated the era of the trans-Atlantic ocean liner.