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 8,588 long tons (8,726 t), approximately 29% of the class's total displacement.
Their waterline armor belt was 305 to 229 millimetres (12 to 9 in) thick; below it was a strake of 102 mm (4 in) armor.
The deck armor ranged in thickness from 32 to 51 mm (1.3 to 2.0 in).
The turrets were protected with an armor thickness of 279.4 mm (11.0 in) on the face, 228.6 mm (9.0 in) on the sides, and 114.5 mm (4.51 in) 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 351 millimetres (13.8 in) thick.
Additionally, the vessels contained 737 watertight compartments (574 underneath the armor deck, 163 above) to preserve buoyancy in the event of battle damage.

During their reconstruction, the armor of the battleships was substantially upgraded.
Their deck armor was increased to a maximum thickness of 114 mm (4.5 in), and a longitudinal bulkhead of 76 mm (3.0 in) of high-tensile steel was added to improve the underwater protection.
This brought the total armor tonnage up to 12,199 long tons (12,395 t), approximately 31% of the total displacement of the Fusō class.
Even after these improvements, the armor was still incapable of withstanding 14-inch shells.
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 8,588 long tons (8,726 t), approximately 29% of the class's total displacement. Their waterline armor belt was 305 to 229 millimetres (12 to 9 in) thick; below it was a strake of 102 mm (4 in) armor. The deck armor ranged in thickness from 32 to 51 mm (1.3 to 2.0 in). The turrets were protected with an armor thickness of 279.4 mm (11.0 in) on the face, 228.6 mm (9.0 in) on the sides, and 114.5 mm (4.51 in) 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 351 millimetres (13.8 in) thick. Additionally, the vessels contained 737 watertight compartments (574 underneath the armor deck, 163 above) to preserve buoyancy in the event of battle damage.
During their reconstruction, the armor of the battleships was substantially upgraded. Their deck armor was increased to a maximum thickness of 114 mm (4.5 in), and a longitudinal bulkhead of 76 mm (3.0 in) of high-tensile steel was added to improve the underwater protection. This brought the total armor tonnage up to 12,199 long tons (12,395 t), approximately 31% of the total displacement of the Fusō class. Even after these improvements, the armor was still incapable of withstanding 14-inch shells.
Fusō-class battleship