Although this mosque is thought to be one of the oldest Islamic buildings on the East African coast, it has been extensively restored and appears relatively modern on the outside. However, on the inside are inscriptions (in Kufic and Arabic) dating to 1107 and 1770.
The mosque is just north of the main beach area in Kizimkazi Dimbani. If you want to take a look, ask permission and (if you're non-Muslim) get a local to accompany you. As in all mosques, you should take off your shoes and cover up bare shoulders or legs.