A thalassocracy is a Greek expression for a state whose realms are primarily marine - an empire at sea, such as the Phoenician network of merchant cities, in no genuine sense an "empire", linked by the sealanes and scarecely penetrating the interior, even in the homeland of Tyre, Sidon and Carthage.

The term (a combination of the Greek "thalassa", "sea", and "kratos", "rule" or "government") was first used by ancient Greeks to describe the government of the Minoan civilization, whose power depended on its navy.

More modern examples include the Republic of Venice, which was conventionally divided in the fifteenth century into the Dogado of Venice and the Lagoon, the Terrafirma of Venetian holdings in northern Italy, and the Mar of the Venetian outlands bound by the sea. The Dubrovnik Republic can also be accounted a "thalassocracy". The Portuguese Empire and the British Empire also started as thalassocracies, but they eventually acquired large land territories.