Ptolemais, coastal city of ancient Cyrenaica (now part of Libya). The site was easily defensible and provided the only safe anchorage between Euhesperides-Berenice (modern Benghazi) and Apollonia (modern Sūsah in Libya). In the 3rd century bc the city received the name Ptolemais from Ptolemy III, who united Cyrenaica with Egypt. Its economy was based on trade with the interior, and the city flourished in Hellenistic times, in the early period of the Roman Empire, and, again, from late in the 3rd century ad, when Diocletian made it the metropolis of the Roman province of Upper Libya.
From the beginning of the 5th century it suffered from raids by the inland Austuriani, as recorded in the letters of Bishop Synesius. Some occupation continued after the Arab conquest (ad 642) to the 11th century.