Isaac ben Judah ibn Ghiyyat (or Ghayyat) (Hebrew: יצחק בן יהודה אבן גיאת, Arabic: ﺇﺑﻦ ﻏﻴﺎث ibn Ghayyath) (1038–1089) was a Spanish rabbi, Biblical commentator, philosopher, and liturgical poet. He was born (H. Graetz cites 1030) and lived in the town of Lucena, where he also headed a rabbinic academy. He died in Cordoba.
According to some authorities he was the teacher of Isaac Alfasi; according to others, his fellow pupil. The best known of his pupils were his son Judah ibn Ghayyat, Joseph ibn Sahl, and Moses ibn Ezra. He was held in great esteem by Samuel ha-Nagid and his son Joseph, and after the latter's death (1066), Ibn Ghayyat was elected to succeed him as rabbi of Lucena, where he officiated until his death.
He was the author of a compendium of ritual laws concerning the festivals, published by Bamberger under the title of Sha'are Simḥah (Fürth, 1862; the laws concerning the Passover were republished by Zamber under the title Hilkot Pesaḥim, Berlin, 1864), and a philosophical commentary on Ecclesiastes, known only through quotations in the works of later authors (Dukes, in Orient, Lit. x. 667-668). The greatest activity of Ibn Ghayyat was in liturgical poetry; he was an author of hundreds of piyyutim, and his hymns are found in the Maḥzor of Tripoli under the title of Sifte Renanot.