D.400 is an east–west state road in southern Turkey.

The 2,057-kilometre (1,278 mi) road starts at Datça in the southwest corner of the Anatolian peninsula and ends at the Iranian border at Esendere.

D.400 runs through the cities of Marmaris, Fethiye, Antalya, Alanya, Mersin, Adana, Gaziantep, Şanlıurfa, and Hakkâri and links to Road 16 in Iran.

Between Nizip and Cizre, D.400 is part of the European route E90.

The table below shows the locations between Datça and Esendere.