Zaidiyya, or Zaidism (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 has many similarities with Sunni.

Its adherents are also known as Fivers.