Propontis

Sea of Marmara Marmara Denizi
Map of the Sea of Marmara
Location	Europe and Asia
Coordinates	40°41′12″N 28°19′7″E / 40.68667°N 28.31861°E / 40.68667; 28.31861Coordinates: 40°41′12″N 28°19′7″E / 40.68667°N 28.31861°E / 40.68667; 28.31861
Type	Inland Sea
Primary inflows	Simav River, Biga Çayı, Nilüfer River
Primary outflows	Turkish Straits
Catchment area	11,500 km2 (4,400 sq mi)
Basin countries	Turkey
Surface area	11,350 km2 (4,380 sq mi)
Average depth	494 m (1,621 ft)
Max. depth	1,370 m (4,490 ft)
Water volume	3,378 km3 (810 cu mi)
Islands	Marmara Island, Avşa, İmralı, Prince Islands, Paşalimanı and Ekinlik Island
Settlements	Istanbul, Bursa, İzmit, Tekirdağ, Balıkesir, Çanakkale, and Yalova

The Sea of Marmara /ˈmɑːrmərə/ (Turkish: Marmara Denizi, Greek: Θάλασσα του Μαρμαρά), also known as the Sea of Marmora or the Marmara Sea, and in the context of classical antiquity as the Propontis (Greek: Προποντίς), is the inland sea, entirely within the borders of Turkey, that connects the Black Sea to the Aegean Sea, thus separating Turkey's Asian and European parts. The Bosphorus strait connects it to the Black Sea and the Dardanelles strait to the Aegean Sea. The former also separates Istanbul into its Asian and European sides. The sea has an area of 11,350 km² (280 km x 80 km) with the greatest depth reaching 1,370 m (4,490 ft).

The sea takes its name from the island of Marmara, which is rich in sources of marble, from the Greek μάρμαρον (marmaron), "marble".

The sea's ancient Greek name Propontis derives from pro- (before) and pontos (sea), deriving from the fact that the Greeks sailed through it to reach the Black Sea, Pontos. In Greek mythology, a storm on Propontis brought the Argonauts back to an island they had left, precipitating a battle where either Jason or Heracles killed King Cyzicus, who mistook them for his Pelasgian enemies.