Arabian Sea, ancient Mare Erythraeum, northwest part of the Indian Ocean, lying between Arabia and India. The Gulf of Aden, extended by the Red Sea, and the Gulf of Oman, extended by the Persian Gulf, are its principal arms. The submarine Carlsberg ridge, SE of Socotra Island, is the sea's southern boundary. The Indus River is the largest stream flowing into the sea. The Arabian Sea has long been an important trade route between India and the West; its chief ports are Aden, Yemen; Karachi, Pakistan; and Mumbai, India.