Al-Mahdi Ahmad ibn Yahya al-Murtada’ (born 1373 – died 1436) was the Imam of the Zaidi state in Yemen from 1391 to 1436 or 1392.
Ahmad was a twelfth generation descendant of the Zaidi imam al-Da’i Yusuf ibn Yahya. He received a very good education and was a prolific writer on various subjects. In 1391 Imam al-Nasir Salah al-Din ibn al-Mahdi died, leaving behind a young progeny. Qadi al-Dawari temporarily took over the administration of the Zaidi state in the name of the sons of al-Nasir. However, the Zaidi scholars met in the Jamal al-Din Mosque in Sana’a and decided to appoint Ahmad ibn Yahya as Imam under the name of al-Mahdi Ahmad. This move was not accepted by al-Dawani, who immediately appointed the son of the deceased imam, al-Mansur Ali ibn Salah al-Din. Al-Mahdi Ahmad and his followers withdrew from Sana’a to Bayt Baws, and for a year the two imams fought for sole control of the imamate. In 1392, al-Mahdi Ahmad was captured by the forces of al-Mansur Ali and imprisoned. In 1399, the former imam escaped with the help of sympathetic prison guards. He lived a private life until his death from the plague in 1436. While he lacked the administrative or military skills necessary for the imamate, al-Mahdi Ahmad left behind a large body of writings on dogmatics, logic, poetry, grammar, and law. For example, he composed the religious jurisprudential encyclopedia Bahr al-Azhar.