Al-Musta'in Billah () was the regnal name of the twelfth Abbasid Caliph ruling from 862 to 866.
The title was also used by:
Sulayman ibn al-Hakam, the fifth caliph of Córdoba (1009–1010, 1013–1016) Al-Musta'in I, the first Hudid ruler of Zaragoza (1039–1046/1049) Al-Musta'in II, the fourth Hudid ruler of Zaragoza (1085–1110) Al-Musta'in (Egypt), the tenth Caliph of Cairo (1406–1414) Abu Nasr Sa'd of Granada, the twentieth sultan of Granada (1455–1462, 1462–1464) Abdullah of Pahang, (born 1959) also known as Al-Sultan Abdullah Ri'ayatuddin Al-Mustafa Billah Shah ibni Almarhum Sultan Haji Ahmad Shah Al-Musta'in Billah, the Malaysian Sultan of Pahang