The Order of Assassins (Arabic: حَشّاشِین, romanized: Ḥashshāshīyīn; Persian: حشاشين, romanized: Ḥaššāšīn) was a Nizari Isma'ili order of assassins that existed between 1090 and 1275 AD, founded by Hasan al-Sabbah.

During that time, the members lived in the mountain castles in Persia and the Levant, and held a strict subterfuge policy throughout the Middle East, posing a substantial strategic threat to Fatimid, Abbasid, and Seljuk authority, and killing several Christian leaders. Over the course of nearly 200 years, they killed hundreds who were considered leading enemies of the Nizari Isma'ili state. The modern term assassination is believed to stem from the tactics used by the Assassins.

Contemporaneous historians include ibn al-Qalanisi, Ali ibn al-Athir, and Ata-Malik Juvayni. The former two referred to the Assassins as batiniyya, an epithet widely accepted by Isma'ilis themselves.

The Assassins were founded by Hassan-i Sabbah. The state was formed in 1090 after the capture of Alamut Castle in the Alborz mountain range of Persia, which served as the Assassins' headquarters. The Alamut and Lambsar castles became the foundation of a network of Isma'ili fortresses throughout Persia and Syria that formed the backbone of Assassin power, and included Syrian strongholds at Masyaf, Abu Qubays, al-Qadmus and al-Kahf. The Western world was introduced to the Assassins by the works of Marco Polo who understood the name as deriving from the word hashish. The Assassins posed a substantial strategic threat to Fatimid, Abbasid, and Seljuk authority. Over the course of nearly 200 years, they killed hundreds – including three caliphs, a ruler of Jerusalem and several Muslim and Christian leaders.

Notable victims of the Assassins include Janah ad-Dawla, emir of Homs, (1103), Mawdud ibn Altuntash, atabeg of Mosul (1113), Fatimid vizier Al-Afdal Shahanshah (1121), Seljuk atabeg Aqsunqur al-Bursuqi (1126), Fatimid caliph al-Amir bi-Ahkami'l-Lah (1130), Taj al-Mulk Buri, atabeg of Damascus (1132), and Abbasid caliphs al-Mustarshid (1135) and ar-Rashid (1138). Saladin, a major foe of the Assassins, escaped assassination twice (1175–1176). The first Frank known to have been killed by the Assassins was Raymond II, Count of Tripoli, in 1152. The Assassins were acknowledged and feared by the Crusaders, losing the de facto King of Jerusalem, Conrad of Montferrat, to an Assassin's blade in 1192 and Lord Philip of Montfort of Tyre in 1270. The Ismaili Assassins were created to revive the Shi'a Ismaili Fatimid Caliphate in Egypt, which had been destroyed by the Sunni Seljuks.

Accounts of the Assassins were preserved within Western, Arabic, Syriac, and Persian sources where they are depicted as trained killers, responsible for the systematic elimination of opposing figures. European orientalists in the 19th and 20th centuries also referred to the Isma'ili Assassins in their works, writing about them based on accounts in seminal works by medieval Arab and Persian authors, particularly ibn al-Qalanisi's Mudhayyal Ta'rikh Dimashq (Continuation of the Chronicle of Damascus), ibn al-Athir's al-Kāmil fit-Tārīkh (The Complete History), and Juvayni's Tarīkh-i Jahān-gushā (History of the World Conqueror). The Order would finally come to an end during the rule of Imam Rukn al-Din Khurshah when the Isma'ili State was eventually destroyed as Khurshah surrendered the castles after the Mongol invasion of Persia. Khurshah died in 1256 and, by 1275, the Mongols had destroyed and eliminated the order of Assassins.

Hassan-i Sabbah was born in Qom, ca. 1050, and did his religious studies in Cairo with the Fatimids. Sabbah's father was a Qahtanite Arab, said to be a descendant of Himyaritic kings, having emigrated to Qom from Kufa. He made his way to Persia where, through subterfuge, he and his followers captured Alamut Castle in 1090. Sabbah adapted the fortress to suit his needs not only for defense from hostile forces, but also for indoctrination of his followers. After laying claim to the fortress at Alamut, Sabbah began expanding his influence outwards to nearby towns and districts, using his agents to gain political favour and to intimidate the local populations. Spending most of his days at Alamut producing religious works and developing doctrines for his order, Sabbah would never again leave his fortress.

Shortly after establishing their headquarters at Alamut Castle, the sect captured Lambsar Castle, to be the largest of the Isma'ili fortresses and confirming the Assassins' power in northern Persia. The estimated date of the capture of Lambsar varies between 1096 and 1102. The castle was taken under the command of Kiya Buzurg Ummid, later Sabbah's successor, who remained commandant of the fortress for twenty years. No interactions between the Christian forces of the First Crusade and the Assassins have been noted, with the latter concentrating on the Muslim enemies of the former. Other than a mention of Tancred's 1106 taking of Apamea (see below) in Gesta Tancredi,