The seas name stems from Caspi, the  ancient people who lived to the southwest of the sea in Transcaucasia. Strabo (died circa AD 24) wrote that "to the country of the Albanians (Caucasian Albania, not to be confused with the country of Albania) belongs also the territory called Caspiane, which was named after the Caspian tribe, as was also the sea; but the tribe has now disappeared". Moreover, the Caspian Gates, part of Irans Tehran province, may evince such people migrated to the south. The Iranian city of Qazvin shares the root of its name with this common name for the sea. The traditional and medieval Arabic name for the sea was Baḥr (sea) Khazar, but in recent centuries the common and standard name in Arabic language has become بحر قزوين Baḥr Qazvin, the Arabized form of Caspian. In modern Russian language, it is known as , Kaspiyskoye more.
Some Turkic ethnic groups refer to it with the Caspi(an) descriptor; in Kazakh it is called , Kaspiy teñizi, , . Others refer to it as the Khazar sea: ; , . In all these the first word refers to the historical Khazar Khaganate, a large empire based to the north of the Caspian Sea between the 7th and 10th centuries.
In Iran, the lake is referred to as the Mazandaran Sea (), after the historic Mazandaran Province at its southern shores.
Old Russian sources use the Khvalyn or Khvalis Sea () after the name of Khwarezmia.
Among Greeks and Persians in classical antiquity it was the Hyrcanian ocean.
Renaissance European maps labelled it as the Abbacuch Sea (Oronce Fines 1531 world map), Mar de Bachu (Ortellius 1570 map), or Mar de Sala (the Mercator 1569 world map).
It was also sometimes called the Kumyk Sea and Tarki Sea (derived from the name of the Kumyks and their historical capital Tarki).