The ship's waterline armor belt was 305 mm (12 in) thick and tapered to a thickness of 100 mm (3.9 in) at its bottom edge; above it was a strake of 229 mm (9.0 in) armor.
The main deck armor was 69 mm (2.7 in) while the lower deck was 75 mm (3 in) thick.
The turrets were protected with an armor thickness of 305 mm on the face, 230–190 mm (9.1–7.5 in) on the sides, and 152–127 mm (6.0–5.0 in) on the roof.
The barbettes of the turrets were protected by armor 305 mm thick, while the casemates of the 140 mm (1.6 in) guns were protected by 25 mm (0.98 in) armor plates.
The sides of the conning tower were 369 mm (14.5 in) thick.

The new 41 cm turrets installed during Nagato's reconstruction were more heavily armored than the original ones.
Face armor was increased to 460 mm (18.1 in), the sides to 280 mm (11.0 in), and the roof to 250–230 mm (9.8–9.1 in).
The armor over the machinery and magazines was increased by 38 mm on the upper deck and 25 mm (0.98 in) on the upper armored deck.
These additions increased the weight of the ship's armor to 13,032 metric tons (12,826 long tons), 32.6 percent of her displacement.
In early 1941, as a preparation for war, Nagato's barbette armor was reinforced with 100 mm (3.9 in) armor plates above the main deck and 215 mm (8.5 in) plates below it.
The ship's waterline armor belt was 305 mm (12 in) thick and tapered to a thickness of 100 mm (3.9 in) at its bottom edge; above it was a strake of 229 mm (9.0 in) armor. The main deck armor was 69 mm (2.7 in) while the lower deck was 75 mm (3 in) thick. The turrets were protected with an armor thickness of 305 mm on the face, 230–190 mm (9.1–7.5 in) on the sides, and 152–127 mm (6.0–5.0 in) on the roof. The barbettes of the turrets were protected by armor 305 mm thick, while the casemates of the 140 mm (1.6 in) guns were protected by 25 mm (0.98 in) armor plates. The sides of the conning tower were 369 mm (14.5 in) thick.
The new 41 cm turrets installed during Nagato's reconstruction were more heavily armored than the original ones. Face armor was increased to 460 mm (18.1 in), the sides to 280 mm (11.0 in), and the roof to 250–230 mm (9.8–9.1 in). The armor over the machinery and magazines was increased by 38 mm on the upper deck and 25 mm (0.98 in) on the upper armored deck. These additions increased the weight of the ship's armor to 13,032 metric tons (12,826 long tons), 32.6 percent of her displacement. In early 1941, as a preparation for war, Nagato's barbette armor was reinforced with 100 mm (3.9 in) armor plates above the main deck and 215 mm (8.5 in) plates below it.
Japanese battleship Nagato