Abu Ali al-Hasan al-Tusi Nizam al-Mulk (نظام الملك، ابو علي الحسن الطوسي in Arabic; 1018 – 14 October 1092) was a celebrated Persian scholar and vizier of the Seljuk Turks.

He was born in the town of Tus in Persia and initially served the Ghaznavid sultans. By 1059 he was the chief administrator of the entire Khorasan province. Later, from 1063, he served the Seljuks as vizier and remained in that position throughout the reigns of Alp Arslan (1063-1072) and Malik Shah I (1072-1092). He left a great impact on organization of the Seljuk governmental bodies and hence the title Nizam al-Mulk which translates as "the order of state".

Aside from his extraordinary influence as vizier with full authority, he is also well-known for systematically founding a number of schools of higher education in several cities, the famous Nizamiyyah schools, which were named after him. In many aspects, these schools turned out to be the predecessors and models of universities that were established in Europe.

Nizam ul-Mulk is also widely known for his voluminous treatise on kingship titled Siyasatnama (The Book of Government). He also wrote a book titled wasiyat nama, written for his son, which is not unsimilar to the famous book of Qabus nama.

He was assassinated by a member of the Hashshashin sect in 1092 near Nahavand, Persia.

Nizam ul-Mulk Tusi was a staunch follower of the Shafi'i school of Sunni Islam.