Abū Yahya Zakariya al-Musta'sim bi'llah (Arabic: أبو يحيى زكريا المستعصم بالله; died 1389) was the eighth Abbasid caliph of Cairo under the tutelage of the Mamluk Sultanate.
Al-Musta'sim was the son of al-Wathiq I (the fourth Caliph of Cairo).
He was first installed in the office in 1377 for a short period of time, after al-Mutawakkil I was deposed.
After few years, in 1386 al-Musta'sim was again installed as caliph of Cairo for the Mamluk Sultanate.
This biography of a member of a Middle Eastern royal house is a stub.