The Decima Flottiglia MAS (Decima Flottiglia Motoscafi Armati Siluranti, also known as La Decima or Xª MAS) (Italian for "10th Torpedo-Armed Motorboat Flotilla") was an Italian flotilla, with marines and commando frogman unit, of the Regia Marina (Royal Italian Navy). The acronym MAS also refers to various light torpedo boats used by the Regia Marina during World War I and World War II.

Decima MAS was active during the Battle of the Mediterranean and took part in a number of daring raids on Allied shipping. These operations involved surface speedboats (such as the raid on Souda Bay), human torpedoes (the raid on Alexandria) and Gamma frogmen (against Gibraltar). During the campaign, Decima MAS took part in more than a dozen operations which sank or damaged five warships (totalling 78,000 tons) and 20 merchant ships (totalling 130,000 GRT).

In 1943, after the Italian dictator Benito Mussolini was ousted, Italy left the Tripartite Pact. Some of the Xª MAS men who were stationed in German-occupied northern and central Italy enlisted to fight for Mussolini's newly formed Italian Social Republic (Repubblica Sociale Italiana or RSI) and retained the unit title, but were primarily employed as an anti-partisan force operating on land. Other Xª MAS men in southern Italy or other Allied-occupied areas joined the Italian Co-Belligerent Navy as part of the Mariassalto (Naval Assault) unit.

In World War I, on 1 November 1918, Raffaele Paolucci and Raffaele Rossetti of the Regia Marina rode a human torpedo (nicknamed Mignatta or "leech") into the harbour of Pula, where they sank the battleship Jugoslavija, of the navy of the State of Slovenes, Croats and Serbs, formerly the Austro-Hungarian battleship SMS Viribus Unitis, and the freighter Wien using limpet mines. They had no underwater breathing sets, and thus had to keep their heads above water to breathe. They were discovered and taken prisoner as they attempted to leave the harbour.

In the 1920s, sport spearfishing without breathing apparatus became popular on the Mediterranean coast of France and Italy. This spurred the development of modern swimfins, diving masks and snorkels.^{[citation needed]}

In the 1930s Italian sport spearfishermen began using industrial or submarine-escape oxygen rebreathers, starting scuba diving in Italy.^{[citation needed]}

This new type of diving came to the attention of the Regia Marina which founded the first special forces underwater frogman unit, later copied by the Royal Navy and United States Navy. Capitano di Fregata (Commander) Paolo Aloisi was the first commander of the 1ª Flottiglia Mezzi d'Assalto ("First Assault Vehicle Flotilla"), formed in 1939 as a result of the research and development efforts of Majors Teseo Tesei and Elios Toschi of the naval combat engineers. The two resurrected Paolucci's and Rossetti's concept of human torpedoes.

In 1941, Commander Vittorio Moccagatta re-organised the First Flotilla into the Decima Flottiglia MAS, and divided the unit into two parts – a surface group operating fast explosive motor boats, and a sub-surface weapons group using human torpedoes called SLC (siluri a lenta corsa or "slow-running torpedoes", but nicknamed Maiale or "Pig" by their crews), as well as "Gamma" assault swimmers (nuotatori) using limpet mines. Moccagatta also created the frogman training school at the San Leopoldo base of the Italian Naval Academy in Livorno.