Princess Royal was armed with eight BL 13.5-inch Mk V guns ("BL" for breech-loading) in four twin hydraulically powered turrets, designated 'A', 'B', 'Q' and 'X' from bow to stern.
Her secondary armament consisted of 16 BL 4-inch Mk VII guns, most of which were mounted in casemates in the superstructure.
The two guns mounted on the deck above the forward group of casemates were fitted with gun shields in 1913 and 1914 to better protect their crews from enemy fire.

The battlecruiser was built without anti-aircraft (AA) guns, but from October 1914 to December 1916 she was fitted with a single QF 6 pounder Hotchkiss gun ("QF" for quick-firing) on a high-angle mount, which had a maximum depression of 8° and a maximum elevation of 60°.
Its 6-pound (2.7 kg) shell was fired at a muzzle velocity of 1,765 ft/s (538 m/s) and a rate of fire of 20 rounds per minute.
It had a maximum ceiling of 10,000 feet (3,000 m), but an effective range of only 3,600 feet (1,100 m).
A single QF 3-inch 20 cwt AA gun was added in January 1915 and carried until April 1917.
Its high-angle mount had a maximum depression of 10° and a maximum elevation of 90°.
The gun fired a 12.5-pound (5.7 kg) shell at a muzzle velocity of 2,500 ft/s (760 m/s) at a rate of 12 to 14 rounds per minute.
It had a maximum effective ceiling of 23,500 ft (7,200 m).

Princess Royal received a fire-control director between mid-1915 and May 1916 that centralised fire-control under the gunnery officer who now fired the guns.
To align their guns on the target, the turret crewmen had to follow pointers whose position was transmitted from the director.
This greatly increased accuracy as it was easier for the director to spot the fall of shells and eliminated the shell spread caused by the ship's roll as the turrets fired individually.

By early 1918, Princess Royal carried a Sopwith Pup and a Sopwith 1½ Strutter on flying-off ramps fitted on top of 'Q' and 'X' turrets.
The Pup was intended to shoot down Zeppelins while the 1½ Strutter was used for spotting and reconnaissance.
Each platform had a canvas hangar to protect the aircraft during inclement weather.
Princess Royal was armed with eight BL 13.5-inch Mk V guns ("BL" for breech-loading) in four twin hydraulically powered turrets, designated 'A', 'B', 'Q' and 'X' from bow to stern. Her secondary armament consisted of 16 BL 4-inch Mk VII guns, most of which were mounted in casemates in the superstructure. The two guns mounted on the deck above the forward group of casemates were fitted with gun shields in 1913 and 1914 to better protect their crews from enemy fire.
The battlecruiser was built without anti-aircraft (AA) guns, but from October 1914 to December 1916 she was fitted with a single QF 6 pounder Hotchkiss gun ("QF" for quick-firing) on a high-angle mount, which had a maximum depression of 8° and a maximum elevation of 60°. Its 6-pound (2.7 kg) shell was fired at a muzzle velocity of 1,765 ft/s (538 m/s) and a rate of fire of 20 rounds per minute. It had a maximum ceiling of 10,000 feet (3,000 m), but an effective range of only 3,600 feet (1,100 m). A single QF 3-inch 20 cwt AA gun was added in January 1915 and carried until April 1917. Its high-angle mount had a maximum depression of 10° and a maximum elevation of 90°. The gun fired a 12.5-pound (5.7 kg) shell at a muzzle velocity of 2,500 ft/s (760 m/s) at a rate of 12 to 14 rounds per minute. It had a maximum effective ceiling of 23,500 ft (7,200 m).
Princess Royal received a fire-control director between mid-1915 and May 1916 that centralised fire-control under the gunnery officer who now fired the guns. To align their guns on the target, the turret crewmen had to follow pointers whose position was transmitted from the director. This greatly increased accuracy as it was easier for the director to spot the fall of shells and eliminated the shell spread caused by the ship's roll as the turrets fired individually.
By early 1918, Princess Royal carried a Sopwith Pup and a Sopwith 1½ Strutter on flying-off ramps fitted on top of 'Q' and 'X' turrets. The Pup was intended to shoot down Zeppelins while the 1½ Strutter was used for spotting and reconnaissance. Each platform had a canvas hangar to protect the aircraft during inclement weather.
HMS Princess Royal (1911)