Saru castles (Persian: قلعه های سارو; also transliterayed as Soru) are two related castles located at 10 km in north east of Semnan, Iran, on mountains of south and north of a valley called Kalâteh Sârū (کلاته سارو, "Saru Castle") or Mazra'eh-ye Sârū (مزرعه سارو, "Saru Farm"). The northern castle, the Lesser Saru (ساروی کوچک), is mostly in ruins. The southern castle, the Greater Saru (ساروی بزرگ), or simply Saru Castle (قلعه سارو), is the main one and is relatively intact. It is 100 m higher than the Lesser Saru. The Greater Saru has triple defensive outer walls and an unusually double-bend main entrance way, and features a sophisticated water catchment area. The Lesser Saru was used to defend the Kalateh Saru natural springs from which water was pumped to the main castle.
Saru castle(s) are believed to be used by Arsacids, Dabuyids of Tabaristan, Buyids, and eventually by the Nizari Ismailis. Saru is relatevely less-researched, and is a well-preserved case to study the architecture of the Ismaili castles. Saru castles are located at 80 km of Gerdkuh, another Ismaili castle of the Qumis region. It is believed that Ismailis built the current castle on the site of an earlier fortification.
The etymology of the word Sârū (سارو) is uncertain.