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 Tamilnadu 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 ("land-based hegemony").
The term thalassocracy can also simply refer to naval supremacy, in either military or commercial senses. The Ancient Greeks first used the word thalassocracy to describe the government of the Minoan civilization, whose power depended on its navy. Herodotus distinguished sea-power from land-power and spoke of the need to counter the Phoenician thalassocracy by developing a Greek "empire of the sea".
Its realization and ideological construct is called maritimism (as in the case of the Estado Novo), contrasting continentalism.
Thalassocracy was a resurrection of a word known from a very specific classical document, which British classical scholar John Linton Myres termed "the List of Thalassocracies". The list was in the Chronicon, a work of universal history of Eusebius, an early 4th century bishop of Caesarea Maritima. Eusebius categorized several historical polities in the Mediterranean as "sea-controlling", and listed them in a chronology.
The list includes a successive series of "thalassocracies", begins from the Lydians after the fall of Troy, and ends with Aegina, each controlled the sea for a number of years. The list therefore presents a series of the successive exclusive naval domains, as the total control of the seas changed hands between these thalassocracies.