Yahya III ibn al-Qasim (Arabic: يحيى الثالث بن القاسم) was an Idrisid ruler of Morocco.

Yahya was the son of al-Qasim, a younger son of the second Idrisid ruler of Morocco, Idris II (r. 808–828).^[1][2] The family of al-Qasim controlled northern Morocco, with the cities of Tangier, Basra and Ceuta,^[2] and for a time in the 860s even the western half of the Idrisid capital, Fes.^[3]

Known by the sobriquet al-Miqdam, Yahya was called upon for assistance by the inhabitants of the Qaraqiyyin quarter of Fes when the town was occupied by the Kharijite rebel Abd al-Razzaq.^[2] Yahya drove the rebels away, and is traditionally accounted as the eighth Idrisid emir, until his death in 905.^[4]

Modern historians on the other hand consider that his rule over Fes is unlikely to have lasted long: his uncle, Dawud, appears to have ruled the city in 877, while numismatic evidence shows that between 880 and 893 the capital was ruled by members of a different branch of the Idrisid dynasty, descending from another uncle, Isa.^[2]

Yahya fell in battle in 905 against the forces of a cousin from a different branch of the dynasty, Yahya IV.^[4]