Qizilbash or Kizilbash were a diverse array of mainly Turkoman Shia militant groups that flourished in Azerbaijan, Anatolia, Kurdistan, the Armenian highlands, and the Caucasus from the late 15th century onwards, and contributed to the foundation of the Safavid dynasty in early modern Iran.

By the 18th-century, anyone involved with the Safavid state—militarily, diplomatically, or administratively—came to be broadly referred to as "Qizilbash". It was eventually applied to some inhabitants of Iran. In the early 19th-century, Shia Muslims from Iran could be referred as "Qizilbash", thus highlighting the influence of the distinctive traits of the Safavids, despite the Iranian shah (king) Fath-Ali Shah Qajar (r. 1797–1834) simultaneously creating a Qajar dynastic identity grounded in the pre-Islamic past.

The word Qizilbash derives from Turkish Qızılbaş, meaning "red head". The expression is derived from their distinctive twelve-gored crimson headwear (tāj or tark in Persian; sometimes specifically titled "Haydar's Crown" تاج حیدر / Tāj-e Ḥaydar), indicating their adherence to the Twelve Imams and to Shaykh Haydar, the spiritual leader (sheikh) of the Safavid order in accordance with the Imamate in Twelver doctrine. The name was originally a pejorative label given to them by their Sunni Ottoman foes, but soon it was adopted as a mark of pride.

The origin of the Qizilbash can be dated from the 15th century onward, when the spiritual grandmaster of the movement, Shaykh Haydar (the head of the Safaviyya Sufi order), organized his followers into militant troops. The Qizilbash were originally composed of seven Azerbaijani-speaking Turkic tribes: Rumlu, Shamlu, Ustajlu, Afshar, Qajar, Tekelu, and Zulkadar (Dulkadirs/Dhu’l-Qadar).

Connections between the Qizilbash and other religious groups and secret societies, such as the Mazdaki movement in the Sasanian Empire, or its more radical offspring, the Khurramites, and Turkic shamanism, have been suggested. Of these, the Khurramites were, like the Qizilbash, an early ghulat group and dressed in red, for which they were termed "the red ones" (Persian: سرخ‌ جامگان, romanized: sarkh jāmegān, Arabic: محمرة, romanized: muḥammirah) by medieval sources. In this context, Turkish scholar Abdülbaki Gölpınarlı sees the Qizilbash as "spiritual descendants of the Khurramites".

The Qizilbash were a coalition of many different tribes of predominantly (but not exclusively) Turkic-speaking background united in their adherence to the Safavid order. Apart from Turkomans, the Qizilbash also included Kurds, Lurs, Persians, and Talysh after Shah Abbas's military reform in the beginning of the 17th century.

As murids (sworn students) of the Safavi pirs (spiritual guides), the Qizilbash owed implicit obedience to their leader in his capacity as their murshid-e kāmil "supreme spiritual director" and, after the establishment of the kingdom, as their padishah (great king). The kingdom's establishment thus changed the purely religious pir–murid relationship into a political one. As a consequence, any act of disobedience of the Qizilbash Sufis against the order of the spiritual grandmaster (Persian: nāsufigari "conduct unbecoming of a Sufi") became "an act of treason against the king and a crime against the state", as was the case in 1614 when Abbas the Great put some followers to death.

The Qizilbash adhered to heterodox Shi’i doctrines encouraged by the early Safavi sheikhs Haydar and his son Ismail I. They regarded their rulers as divine figures, and so were classified as ghulat "extremists" by orthodox Twelvers.