The maritime republics, also called merchant republics, were Italian thalassocratic port cities which, starting from the Middle Ages, enjoyed political autonomy and economic prosperity brought about by their maritime activities. The term, coined during the 19th century, generally refers to four Italian cities, whose coats of arms have been shown since 1947 on the flags of the Italian Navy and the Italian Merchant Navy: Amalfi, Genoa, Pisa, and Venice. In addition to the four best known cities, Ancona, Gaeta, Noli, and, in Dalmatia, Ragusa, are also considered maritime republics; in certain historical periods, they had no secondary importance compared to some of the better known cities.

A thalassocracy or thalattocracy, sometimes also maritime empire, is a state with primarily maritime realms, an empire at sea, or a seaborne empire. Traditional thalassocracies seldom dominate interiors, even in their home territories. Examples of this were the Phoenician states of Tyre, Sidon and Carthage; the Italian maritime republics of Venice and Genoa of the Mediterranean; the Chola dynasty of Tamil Nadu in India; the Omani Empire of Arabia; and the Austronesian empires of Srivijaya and Majapahit in Maritime Southeast Asia. Thalassocracies can thus be distinguished from traditional empires, where a state's territories, though possibly linked principally or solely by the sea lanes, generally extend into mainland interiors in a tellurocracy.