The book is dedicated to the Ionian Sea, which is part of the Mediterranean. The encyclopedia contains about 600 articles on the hydrographic and geographic objects, hydrological features of the sea, biological resources, as well as administrative-territorial units of the Ionian countries. The most significant natural objects like islands, peninsulas, bays, rivers, mountains, their geographical peculiarities are briefly described as well as economy, culture and history, cities, ports, international agreements, research institutions, activities of outstanding scientists, researchers, travelers are presented in the publication. The chronology of the main historical events that have become significant landmarks in the history of discovery and exploration of the Ionian Sea from the 31 B.C. to the present day is given.