Zaidiyya, or Zaidism (Arabic: الزيدية az-zaydiyya, adjective form Zaidi or Zaydi) is a Shi'a Muslim school of thought named after Zayd ibn ʻAlī, the grandson of Husayn ibn ʻAlī. Followers of the Zaydi Islamic jurisprudence are called Zaydi Shi'a and are particularly prevalent in Yemen. The Zaydi Shi'a have a unique approach within Shi'a Islamic thought that renders similarities with orthodox Sunni Islam, to the extent that some adherents of the majority Twelver sect of Shi'ism even refer to it as the fifth school of Sunni  jurisprudence.

The Zaydis, Twelvers and Ismailis recognize the same first four Imāms of Shi'a Islam, however, the Zaydis recognize Zayd ibn Ali as the Fifth Imām. After Zayd ibn Ali, the Zaydis recognize other descendants of Hasan ibn ʻAlī or Husayn ibn ʻAlī as their Imāms. Other well known Zaydi Imāms in history were Yahya ibn Zayd, Muhammad al Nafs az-Zakiyah and Ibrahim ibn Abdullah.

In matters of Islamic jurisprudence, the Zaydis follow Zayd ibn Ali's teachings which are documented in his book Majmu'l Fiqh (in Arabic: مجموع الفِقه). The Zaydi fiqh is similar to the Sunni Hanafi school of Islamic jurisprudence.