In the Roman Empire, Mare Nostrum was a term that referred to the Mediterranean Sea. Meaning "Our Sea" in Latin, it denoted the body of water in the context of borders and policy; Rome remains the only state in history to have controlled the entire Mediterranean coast. The phrase would have been pronounced [ˈma.rɛ ˈnɔs.t̪rʊ̃ː] in Classical Latin, while it is pronounced [ˈmaː.rɛ ˈnɔs.t̪rum] in Ecclesiastical Latin.

The concept of "Mare Nostrum" is noteworthy for having risen to political significance after the unification of Italy in the 19th century, particularly among Italian nationalists and classical fascists, who attempted to revive the term with the connotation of Italy (and the Italian Empire shortly thereafter) being Rome's successor.

The term Mare Nostrum originally was used by the Romans to refer to the Tyrrhenian Sea after their conquest of Sicily, Sardinia and Corsica during the Punic Wars with Carthage. By 30 BC, Roman dominion had extended from the Iberian Peninsula to Egypt, and Mare Nostrum began to be used in the context of the whole Mediterranean Sea. Other names were also employed, including Mare Internum ("Internal Sea"), but they did not include Mare Mediterraneum, which was a Late Latin creation that was attested to only well after the Fall of Rome.

In the decades following the 1861 unification of Italy, Italian nationalists who saw Italy as the successor state to the Roman Empire attempted to revive the term. In particular, the rise of Italian nationalism during the "Scramble for Africa" of the 1880s led to calls for the establishment of an Italian colonial empire, which introduced for the first time a renewed and modern concept of Mare Nostrum:

Even if the coast of Tripoli were a desert, even if it would not support one peasant or one Italian business firm, we still need to take it to avoid being suffocated in Mare Nostrum.

— Emilio Lupi

The term was again taken up by Benito Mussolini for use in fascist propaganda, in a similar manner to Adolf Hitler's Lebensraum. Mussolini wanted to re-establish the greatness of the Roman Empire and believed that Italy was the most powerful of the Mediterranean countries after World War I. He declared that "the twentieth century will be a century of Italian power" and created one of the most powerful navies of the world in order to control the Mediterranean Sea.

When World War II started, Italy was already a major Mediterranean power that controlled the north and south shores of the central basin. After the fall of France removed the main threat from the west, the British Mediterranean Fleet, with bases in UK-controlled Gibraltar, Malta, Cyprus, Egypt, and Mandatory Palestine remained the only threat to Italian naval power in the Mediterranean. The invasions of Albania, Greece, Yugoslavia and Egypt, and the Siege of Malta sought to extend Axis control over the Sea. This policy was so far-reaching and potentially intrusive, that it threatened neutral nations like Turkey, a threat that İsmet İnönü, the president of Turkey at the time of war, countered by promising to enter the war only if the Soviet Union joined the Allies.