Nasir Khusraw (Persian: ناصرخسرو; 1004 – between 1072–1088) was an Isma'ili poet, philosopher, traveler, and missionary (da'i and later Ḥujjah of Imam al-Mustansir Billah) for the Isma'ili Fatimid Caliphate.

Despite being one of the most prominent Isma'ili philosophers and theologians of the Fatimids and the writer of many philosophical works intended for only the inner circle of the Isma'ili community, Nasir is best known to the general public as a poet and writer who ardently supported his native Persian tongue as an artistic and scientific language. All of Nasir's philosophical Isma'ili works are in Persian, a rarity in the Isma'ili literature of the Fatimids, which primarily used Arabic.

Nasir was a key figure in the spread of Isma'ilism in Central Asia. He is with great reverence called "Pir" or "Shah Sayyid Nasir" by the Isma'ili community of Badakhshan (split between Afghanistan and Tajikistan) and their branches in northern Pakistan, who all consider him to be their founder.

Nasir Khusraw's full name was Abu Mu'in Hamid al-Din Nasir ibn Khusraw ibn Harith al-Qubadiyani al-Marvazi, which he would generally refer to himself by in his prose philosophical works. In his book Safarnama, he mostly calls himself "Nasir", and in his poetry he mostly uses the pen-name "Hujjat", a title meaning "proof", which he had received by the Isma'ili missionary organisation in Cairo. In his poetry he also sometimes use the names of "Nasir" and "Khusraw", the latter being a Persian name, which according to historian Dr C. Alice Hunsberger "would certainly have been a point of great pride to this defender of Persian culture and language." His name is also transliterated as Nasir-i Khusraw and Naser-e Khosrow.

Nasir is the subject of many tales, including a fake autobiography that bears his name and has been passed around between Isma'ilis and non-Isma'ilis for many centuries. The writings of Nasir that are still in existence offer insightful information about his life and philosophy. Yet, it appears that the majority of these manuscripts were censored to remove any references to Isma'ilism by antagonistic Sunni scribes. A native Persian, Nasir was from a family of government bureaucrats and landowners. He most likely adhered to the Twelver Shi'ism form of Islam before his conversion to Isma'ilism. He was born in 1004 in Qubadiyan, a neighborhood of the city of Balkh, which was part of the Marw province in the Khurasan region. The area was then controlled by the Ghaznavids, a culturally Persianised dynasty of Turkic slave origin.

Nasir supposedly started his career as a scribe before moving to the city of Marw to work as a financial administrator. He was allowed entry to the court at Balkh until 1040, when Khurasan was conquered by the Seljuks. He retained his office under the Seljuks, whose co-founder Chaghri Beg ruled Khurasan. Around the age of 41, Nasir underwent a tremendous and sudden change in his spirituality, which drastically altered the direction of his life. According to Nasir, the incident had the characteristics of a dream. He subsequently resigned from his office and converted to Isma'ilism.

In December 1045, Nasir decided to embark on a prolonged journey with the ultimate goal of performing a pilgrimage to Mecca. In March 1046, along with his brother Abu Sa'id and an Indian attendant, he started his journey, which would first end after nearly seven years. From Khurasan, he travelled west, passing through northern and western Iran, Armenia, and Asia Minor. He then descended to Syria, Palestine, and then Arabia, where he performed his pilgrimage. On August 1047, he went to Cairo in Egypt, which served as the capital of the Isma'ili Fatimid Caliphate.

The Fatimid Caliphate was the first and last significant Shi'i dominion until the emergence of the Safavid dynasty of Iran in 1501, which declared Twelver Shi'ism as its official religion. The majority of other governmental power bases were overwhelmingly Sunni, including the Ghaznavid and Seljuk rulers, who demonstrated their support for the Abbasid caliph in Baghdad by making determined efforts to put an end to Isma'ili activity, especially that of Isma'ili missionaries operating under the Fatimid caliph. At Cairo, Nasir was taught Isma'ili teachings, law, and administration by prominent scholars. He met al-Mu'ayyad fi'l-Din al-Shirazi (died 1087), an Isma'ili scholar from Shiraz in southwestern Iran, who had also recently arrived to Cairo. He became Nasir's teacher, instructing him in enigmatic Isma'ili concepts and philosophy.