 Diderot's main battery consisted of four 305mm / 45 Modèle 1906 guns mounted in two twin gun turrets, one forward and one aft. The secondary battery consisted of twelve 240mm / 50 Modèle 1902 guns in twin turrets, three on each side of the ship. A number of smaller guns were carried for defense against torpedo boats. These included sixteen 75 mm (3.0 in) L / 65 guns and ten 47 mm (1.9 in) Hotchkiss guns. The ship was also armed with two submerged 450 mm (17.7 in) torpedo tubes. The ship's waterline armor belt was 270 mm (10.6 in) thick and the main battery was protected by up to 300 mm (11.8 in) of armor. The conning tower also had 300 mm thick sides.
