Persian vizier of the Turkish Seljuq sultans (1063–92), best remembered for his large treatise on kingship, Seyāsat-nāmeh (The Book of Government; or Rules for Kings).

Niẓām al-Mulk was the son of a revenue official for the Ghaznavid Dynasty. Through his father's position, he was born into the literate, cultured milieu of the Persian administrative class. In the years of confusion following the initial Seljuq Turk expansion, his father fled, eventually to Ghazna (now in Afghanistan), where Niẓām al-Mulk, too, in due course entered Ghaznavid service.

He soon returned to Khorāsān, however, and joined the service of Alp-Arslan, who was then the Seljuq governor of Khorāsān. When Alp-Arslan's vizier died, Niẓām al-Mulk was appointed to succeed him, and, when Alp-Arslan himself succeeded his father in 1059, Niẓām al-Mulk had the entire administration of Khorāsān in his hands. His abilities so pleased his master that, when Alp-Arslan became the supreme overlord of the Seljuq rulers in 1063, Niẓām al-Mulk was made vizier.

For the next 30 years, under two remarkable rulers, he occupied this position in an empire that stretched from the Oxus River (now Amu Darya) in the east to Khwārezm and the southern Caucasus and westward into central Anatolia. During these decades, the Seljuq empire was at its zenith; Niẓām al-Mulk's influence guided the sultan's decisions, sometimes even military ones, and his firm control of the central and provincial administration, through his numerous dependents and relatives, implemented those decisions. His aim, like that of other great Persian viziers, was to impress on his less-sophisticated Turkmen rulers, brought up in the rude tradition of the steppe, the superiority of Persian civilization and its political wisdom. His influence was especially felt in the rule of Sultan Malik-Shāh, who succeeded to the Seljuq throne when he was only 18. Such was Niẓām al-Mulk's reputation among contemporaries that he was compared to the Barmakids, viziers to the 8th-century caliph Hārūn al-Rashīd.

Shortly before his death and at Malik-Shāh's request, Niẓām al-Mulk wrote down his views on government in the Seyāsat-nāmeh. In this remarkable work, he barely refers to the organization of the dewan (administration) because he had been able, with the help of his well-chosen servants, to control and model it on traditional lines. But he never had the same power in the dargāh (court) and found much to criticize in the sultan's careless disregard for protocol, the lack of magnificence in his court, the decline in prestige of important officials, and the neglect of the intelligence service. The most severe criticisms in the Seyāsat-nāmeh, however, are of those with heterodox religious views, the Shīʿites in general and the Ismāʿīlītes (Ismāʿīlīte) in particular, to whom he devotes his last 11 chapters. His support of “right religion,” Sunnī (Sunnite) Islām, was not only for reasons of state but also a matter of passionate conviction.

Niẓām al-Mulk expressed his religious devotion in ways that contributed to the Sunnite revival. He founded Niẓāmīyah madrasahs (madrasah) (colleges of higher learning) in many major towns throughout the empire to combat Shīʿite propaganda, as well as to provide reliable, competent administrators, schooled in his own branch of Islāmic law. Less orthodox religious communities among the Ṣūfī orders also benefited from his generosity; hospices, pensions for the poor, and extensive public works related to the pilgrimage to Mecca and Medina were created or sustained by his patronage. Particularly in his last years, when the Ismāʿīlīte threat grew stronger and its partisans found a refuge in Alamūt, the castle of the Assassins, he set himself the task of combating their influence by every means possible.

On Alp-Arslan's death in 1073, Niẓām al-Mulk was left with wider powers, since the late sultan's successor, Malik-Shāh, was only a youth. By 1080, however, Malik-Shāh had become less acquiescent. Niẓām al-Mulk also antagonized the sultan's favourite courtier, Tāj al-Mulk, and he made an enemy of the sultan's wife Terkhen-Khatun by preferring the son of another wife for the succession.

Niẓām al-Mulk was assassinated in 1092, on the road from Eṣfahān to Baghdad, near Nehāvand. The murder was probably committed by an Ismāʿīlīte from Alamūt, possibly with the complicity of Tāj al-Mulk and Terkhen-Khatun, if not that of Malik-Shāh himself. Within a month, however, the sultan, too, was dead, and the disintegration of the great Seljuq empire had begun.

As a great Iranian vizier, Niẓām al-Mulk conspicuously exemplifies the chief minister's role of mediator between a despot, in this instance an alien Turk, and his Persian subjects. Niẓām al-Mulk kept Turkmen immigrants, who had entered Iran with the Seljuqs, engaged in hostilities outside the country; and he tempered military harshness with lessons in judicious clemency and conciliation. He built up Seljuq power with the sultan as the keystone of an integrated administration, and he encouraged the recognition of local rulers as honourable vassals. Niẓām al-Mulk was, for contemporaries, as he has remained for successive generations who read his Seyāsat-nāmeh, the quintessential vizier—wise, prudent, resourceful and successful, and a devout Muslim. By his life and work, he brought the Persian and the Islāmic cultures toward a closer integration at a time when medieval Islām reached its zenith.

The only complete English translation of Niẓām al-Mulk's Seyāsat-nāmeh is by Hubert Darke, The Book of Government; or, Rules for Kings (1960).