| al-Mustamsik المستمسك بالله | |
| Succession: | 16th Caliph of Cairo (first period) |
| Reign: | 27 September 1497–1508 |
| Predecessor: | al-Mutawakkil II |
| Successor: | al-Mutawakkil III |
| Succession1: | (second period) |
| Reign1: | 1516–1517 |
| Predecessor1: | al-Mutawakkil III |
| Successor1: | al-Mutawakkil III |
| Birth Date: | 1470s |
| Birth Place: | Cairo, Mamluk Sultanate (now Egypt) |
| Death Date: | 1521 |
| Death Place: | Ottoman Empire |
| Religion: | Islam |
| Issue: | al-Mutawakkil III |
Al-Mustamsik (; died 1521) was the sixteenth and penultimate Abbasid caliph of Cairo under the tutelage of the Mamluk Sultanate. He served as caliph twice, his first term from 1497 to 1508 and his second term from 1516 to 1517, when he abdicated the position to his son, al-Mutawakkil III.