Eugenius I (sometimes Eugene; died 647) was Archbishop of Toledo from 636 to 646. He became successor in 636 of Justus in that metropolitan see. Like his predecessor he had been a disciple of Helladius in the monastery of Agali. He is famous as an astronomer and astronomical mathematician. As a bishop he was virtuous and intelligent. At this period, under the Visigothic Kingdom, the councils of Toledo were national diets convoked by the monarch, attended by lay lords; they regulated, to some extent, not only spiritual but temporal affairs.