Abdallah ibn al-Mu'izz (died 8 February 975) was the son and heir-apparent of the fourth Fatimid caliph, al-Mu'izz, but died before him.
Abdallah was the second oldest of al-Mu'izz's sons, and was designated heir in 972, as part of the preparations for the move of the Fatimid court from Ifriqiya to recently conquered Egypt. The aim was probably to safeguard the succession, against what was expected to be a difficult and perilous journey. The designation had initially been communicated to a select few senior officials and kept secret even from Abdallah himself, but the elderly majordomo Jawdhar revealed the secret when he performed obeisance to Abdallah before the assembled court. The disillusioned eldest son, Tamim, was then involved in an unsuccessful conspiracy with the son of the Kalbid Emir of Sicily.
Initially this appointment was not widely publicized beyond the court, but Abdallah's name was proclaimed publicly alongside his father's after their arrival in Egypt in June 973. In April 974, during the Qarmatian invasion of Egypt, Abdallah was put in command of the Fatimid army and led it to a decisive victory, that forced the Qarmatians to withdraw back to their home in Bahrayn. In the aftermath of the victory, on 26 May, the victorious Abdallah made a triumphal entrance into Cairo, accompanied by the most prominent captives on the backs of camels, and several thousand severed heads of the Qarmatian fallen impaled on lances; his position as heir apparent was further underlined by riding under the caliphal parasol, the . Abdallah predeceased his father, dying after a brief illness on 8 February 975. Abdallah's younger brother, Nizar, was presented to the court as the designated heir a few days before al-Mu'izz died in December of the same year, and even ruled for several months after his father's death in the latter's name, before finally publicly announcing al-Mu'izz's death and his own ascension to the throne as Caliph al-Aziz .
Abdallah was the last Fatimid prince or heir apparent to be given important military commands, especially as most subsequent Fatimid caliphs ascended the throne as children. Abdallah had at least one son, whose name is unknown, but who unsuccessfully conspired with Sitt al-Mulk to usurp the throne from the underage al-Hakim in 996, and ended his life in prison. A daughter, named Amina and known by the sobriquet Ruqya, later became the concubine of al-Hakim and mother of his successor, al-Zahir .