When the "Fusō" class was completed, the ships' armor was "typical for a pre-Jutland battleship". As built, the armor accounted for a displacement of , approximately 29% of the class's total displacement. Their waterline armor belt was thick; below it was a strake of armor. The deck armor ranged in thickness from . The turrets were protected with an armor thickness of on the face, on the sides, and on the roof. The barbettes of the turrets were protected by armor 305 mm thick, while the casemates of the 152 mm guns were protected by 152 mm armor plates. The sides of the conning tower were thick. Additionally, the vessels contained 737 watertight compartments (574 underneath the armor deck, 163 above) to preserve buoyancy in the event of battle damage.