Aëtius of Amida was a Byzantine physician and medical writer who lived and worked in the 6th century CE. He was born in the city of Amida in the eastern Roman Empire and served as a court physician to Emperor Justinian I in Constantinople. He was likely educated in the Alexandrian medical tradition, which focused on the teachings of earlier authorities like Hippocrates and Galen.

He is known for a single, massive work called the Libri Medicinales, sometimes referred to as the Tetrabiblos. This encyclopedia is organized into 16 books covering a wide range of medical topics, from general diseases and fevers to specialized areas like eye conditions, skin ailments, and gynecology. The work is not an original treatise but a detailed compilation of knowledge from many earlier Greek and Roman medical writers, and the full text survives.

Aëtius's primary historical importance lies in his role as a preserver of knowledge. His encyclopedia is considered one of the three major medical compilations from late antiquity. It is a crucial source for historians because it contains excerpts and ideas from many earlier physicians whose own works have been lost. His detailed sections on women's health were particularly influential for later medical traditions in the Byzantine, Arabic, and Renaissance worlds. His work exemplifies the late antique shift toward systematically organizing existing knowledge for practical use.