克特西亚斯（Κτήσιας，Ctesias），按罗氏希腊拉丁文译音表应译“克忒西阿斯”。前5世纪尼多斯（Cnidus）人，亚达薛西二世御医，历史学家。

克特西亚斯出生于小亚细亚尼多斯行医世家（Asclepiadae），是波斯阿契美尼德王朝亚达薛西二世王室的御医。西西里的狄奥多罗斯与策策斯记载克特西亚斯在担任御医十七年后，于前398年退休回到家乡。因此他应于前415年开始服侍波斯王。据西西里的狄奥多罗斯所述（ii.32），克特西亚斯在被亚达薛西二世俘虏后因其医术而被召成为御医。色诺芬记载克特西亚斯在库纳克萨之战时随从亚达薛西。

For the beetle genus, see Ctesias (beetle).

Ctesias (/ˈtiːʒəs/; Ancient Greek: Κτησίας, Ktēsíās), also known as Ctesias the Cnidian or Ctesias of Cnidus, was a Greek physician and historian from the town of Cnidus in Caria. Ctesias, who lived in the 5th century BC, was physician to Artaxerxes Mnemon, whom he accompanied in 401 BC on his expedition against his brother Cyrus the Younger.