Saru Castles (Persian: قلعه‌های سارو; also spelled Soru, Persian pronunciation: [sɒːˈɾuː]) are two related fortifications located at 10 km in north east of the city of Semnan, Iran, on mountains of south and north of a valley called Kalāteh Sārū (کلاته سارو, literally "Saru Castle") or Mazra'eh-ye Sārū (مزرعه سارو, "Saru Farm").

The northern castle, the Lesser Saru (ساروی کوچک), is mostly in ruins.

[1] The Greater Saru has triple defensive outer walls and an unusual double-bend main entrance way, and features a sophisticated water catchment area.

The Saru Castles are believed to be used by Arsacids, Dabuyids of Tabaristan, Buyids, and eventually by the Nizari Ismailis,[1] who built the current castle on the site of an earlier fortification.

[2] Saru is relatively understudied, and is a well-preserved case to study the architecture of the Ismaili castles.