Abū Manṣūr Nizār al-Muṣṭafá liDīnillāh (1045-1097, Arabic: أبومنصور نزار المصطفى لدين الله‎) was a Fatimid Caliph and a Nizāri Ismā‘ilī Imām. He was deposed by his brother, Aḥmad al-Musta‘lī but his son, al-Hādī ibn Nizār, escaped to Alamūt and took refuge with believers there, thereby continuing the Imāmate.

The followers of Nizār's descendants constitute the majority of the Ismā‘ilī today, with the smaller Musta‘lī branch accepting his younger brother who overthrew him and the Druze ending the Imāmate before either of them.

In his "History of the Ismailis", A.S. Picklay says, "Although Nizar was the rightful claimant to the throne after his father's death, his younger brother Aḥmad al-Musta‘lī, supported by his father-in-law, the chief Vizier Badr al-Jamali, usurped all the power." He further writes, "Mustaali, feeling insecure during Nizar's existence, plotted against Imam Nizar and finally succeeded in making him a prisoner along with his two sons."

In Egypt, Imam Nizar continued his struggle up to 490 A.H., when he was killed. There have been some incorrect theories that Imam Nizar came to Alamut. Actually, he did not, but his son and successor, Imam Hadi, was brought to Alamut from Egypt by Abdul Hasan Saidi, a trusted Da'i of Imam Nizar. Thus the Egyptian period of Ismaili Imams came to an end.