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 Omani Empire of Arabia; and the 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 sometimes called maritimism (cf. Pluricontinentalism or Atlanticism), contrasting continentalism (cf. Eurasianism).

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. Since it does not mention Aegina's final submission of its naval force to Athens, the original list was likely compiled before the consolidation of the Athenian-led Delian League.

Eusebius' list survived through fragments of Diodorus Siculus' works, while also appeared in 4th-century theologian and historian Jerome's Chronicon, and Byzantine chronicler George Syncellus' Extract of Chronography. German classical scholar Christian Gottlob Heyne reconstructed the list through fragments in 1771. The list was then further surveyed by John Myres in 1906-07 and extensively studied by Molly Miller in the 1970s.

The Austronesian peoples of Maritime Southeast Asia developed the Indian Ocean's first true maritime trade network. They established trade routes with Southern India and Sri Lanka as early as 1500 BC, ushering in an exchange of material culture (like catamarans, outrigger boats, lashed-lug and sewn-plank boats, and paan) and cultigens (like coconuts, sandalwood, bananas, and sugarcane); as well as connecting the material cultures of India and China. Indonesians in particular traded in spices (mainly cinnamon and cassia) with East Africa, using catamaran and outrigger boats and sailing with the help of the Westerlies in the Indian Ocean. This trade network expanded west to Africa and the Arabian Peninsula, resulting in the Austronesian colonization of Madagascar by the first half of the first millennium AD. It continued into historic times, later becoming the Maritime Silk Road.

The first thalassocracies in the Indo-Pacific region began to emerge around the 2nd century AD, through the rise of emporia exploiting the prosperous trade routes between Funan and India through the Malacca Strait using advanced Austronesian sailing technologies. Numerous coastal city-states emerged, centered on trading ports built near or around river mouths which allowed easy access to goods from inland for maritime trade. These city-states established commercial networks with other trading centers in Southeast Asia and beyond. Their rulers also gradually Indianized by adopting the social structures and religions of India to consolidate their power.