Al-Mahdi Salah ad-Din (died 1445) was a claimant for the Zaidi state in Yemen, whose tenure as imam is counted from 1436 to 1445.
Salah ad-Din bin Ali was a descendant of the imam al-Mansur Yahya (d. 976) in the thirteenth generation.
His rivals were al-Mansur an-Nasir and al-Mutawakkil al-Mutahhar who belonged to other Rassid branches.
Al-Mansur an-Nasir, who had family ties with the deceased imam, became dominant among the Zaydiyyah community in highland Yemen.
He eventually captured al-Mahdi Salah ad-Din, who died in prison in Kawkaban in 1445.