Zaydism (Arabic: الزَّيْدِيَّة, romanized: az-Zaydiyya), also referred to as Fiver Shi'ism, is a branch of Shia Islam that emerged in the eighth century following Zayd ibn Ali's unsuccessful rebellion against the Umayyad Caliphate. Zaydism is one of the three main branches of Shi'ism, the other two being Twelverism and Ismailism.

Zaydism is typically considered the Shia branch that is closest to Sunni Islam, although the "classical" form of Zaydism (usually referred to as Hadawi) historically changed its stance on Sunni and Shia traditions multiple times, to the point where Zaydis' simply accepting Ali as a rightful successor to Muhammad was enough to consider them Shia.

Zaydis regard rationalism as more important than Quranic literalism and historically were quite tolerant towards Shafi'i Sunnism, the jurisprudential school of about half of the Yemenis. Most of the world's Zaydis are located in northwestern Yemen and Najran in Saudi Arabia.

In the 7th century some early Muslims expected Ali to become the first caliph, successor to Muhammad. After the ascension of Abu Bakr, supporters of Ali (and future Shia) continued to believe only people from Muhammad's family qualify as rulers. They selected an imam from each generation of Muhammad's family. (The proto-Sunni, in contrast, recognized Abu Bakr as a legitimate first caliph). The Zaydis emerged in reverence of Zayd ibn Ali's failed uprising against the Umayyad caliph Hisham ibn Abd al-Malik (r. 724–743). While a majority of the early Shia recognized Zayd's brother, Muhammad al-Baqir, as the fifth leader, some considered Zayd as the fifth imam, and thus in the 8th century formed the Zaydi or "Fivers" offshoot of Islam.

The Zaydis formed the states in what is now known as northern Iran (Tabaristan, 864 CE, by Hasan ibn Zayd, expanded to Daylam and Gilan) and later in Yemen (893 CE, by al-Hadi ila'l-Haqq Yahya). The Zaydis on the Caspian Sea were forcefully converted to Twelver Shi'ism in the 16th century.

The Zaydis in Yemen had initially lived in the highlands and the northern territories, but the extent of their dominance away from their capital of 7 centuries, Saada, had been changing over time. Rassid dynasty was established after an Ottoman invasion in the 16th century. After another interaction with Ottomans, a new succession line was started in the 19th century by Muhammad bin Yahya Hamid ad-Din. With minor interruptions, these two dynasties ruled in Yemen until the creation of Yemen Arab Republic in 1962. While the rulers ostensibly conformed to Hadawi law (thus the "imamate"), the doctrines had to be modified to allow hereditary, as opposed to traditional merit-based, selection of imams.

The end of imam rule in 1962, with the new rulers in Yemen no longer conforming to the requirements of Zaydism, caused Zaydi scholars to call for the restoration of the imamate. This contributed to the North Yemen Civil War that lasted from 1962 to 1970. The national reconciliation of 1970 paused the fighting with traumatized Zaydis following three main routes:

In matters of Islamic jurisprudence, the Zaydis follow Zayd Ibn 'Ali's teachings, which are documented in his book Majmu' Al-Fiqh (Arabic: مجموع الفِقه). Zaydi fiqh is similar to the Hanafi school of Sunni Islamic jurisprudence, as well as the Ibadi school. Abu Hanifa, the founder of the Hanafi school, was favorable and even donated towards the Zaydi cause. Zaydis dismiss religious dissimulation (taqiyya). Zaydism does not rely heavily on hadith, but uses those that are consistent with the Qur'an, and is open to hadith. Some sources argue that Zaydism is simply a philosophy of political government that justifies the overthrow of unjust rulers and prioritizes those who are Ahl al-Bayt.