| Abu Yahya Zakariya al-Musta'sim أبو يحيى زكريا المستعصم | |
| Succession: | 8th Caliph of Cairo (first period) |
| Reign: | 1377 |
| Predecessor: | al-Mutawakkil I |
| Succession1: | (second period) |
| Reign1: | 1386–1389 |
| Successor1: | al-Mutawakkil I |
| Birth Date: | unknown date |
| Birth Place: | Cairo, Mamluk Sultanate (now Egypt) |
| Death Date: | 1389 |
| Death Place: | Cairo, Mamluk Sultanate |
| Religion: | Sunni Islam |
Abū Yahya Zakariya al-Musta'sim bi'llah (; died 1389) was the eighth Abbasid caliph of Cairo under the tutelage of the Mamluk Sultanate. He served twice: first in 1377, then again in 1386–1389.
Al-Musta'sim was the son of al-Wathiq I (the fourth Caliph of Cairo). His given name was Zakariya and his Kunya was Abu Yahya. He was first installed in the office in 1377 for a short period of time, after al-Mutawakkil I was deposed. Al-Musta'sim was also deposed by Mamluk sultan and al-Mutawakkil I was again installed as caliph of Cairo by Mamluks.
After few years, in 1386 al-Musta'sim was again installed as caliph of Cairo for the Mamluk Sultanate. He remained in the office from 1386 to 1389 and he was again succeeded by al-Mutawakkil I in 1389.