Dardanelles (Turkish Çanakkale Boğazi, ancient Hellespont), strait in northwestern Turkey, 70 km (40 mi) long and 2 to 6 km (1 to 4 mi) wide, situated between Asian Turkey and the Gallipoli (Gelibolu) Peninsula of European Turkey. It connects the Aegean Sea and the Sea of Marmara, thus forming a link in the waterway between the Mediterranean and Black seas. The strait constitutes a part of the boundary between Europe and Asia. Important towns on the Dardanelles include Gallipoli on the European side, at the entrance to the Sea of Marmara, and Çanakkale in Asian Turkey. The strait has a swift surface current in the direction of the Aegean Sea; this current is made swifter by seasonal winds. The entrances to the Dardanelles are fortified.

The Dardanelles is prominent in Greek legend and has been famous since early times. The ancient name, Hellespont, is said to have been derived from Helle, who was drowned in the strait when she fell from the back of the ram Chrysomallus. Leander also supposedly perished in the Hellespont during one of his nightly crossings to visit his beloved Hero. The legend inspired the British poet Lord Byron to swim the strait in 1810.

The strategic importance of the Dardanelles was demonstrated in 480 bc by Xerxes I, king of Persia, who, on his way to attack the Greeks, built a bridge of boats across the strait. Control of the Dardanelles in succeeding centuries was a source of power and wealth for those who held it.