The Indefatigable class were the second class built of British battlecruisers which served in the Royal Navy and the Royal Australian Navy during World War I. The design represented a modest reworking of the preceding Invincible class, featuring increased endurance and an improved cross-deck arc of fire for their midships wing turrets achieved by a lengthening of the hull. Like its predecessor, the design resembled the contemporary dreadnought of the Royal Navy, but sacrificed armour protection and one turret from the main battery for a 4-knot speed advantage.

The battlecruiser was a type of capital ship of the first half of the 20th century. These were similar in displacement, armament and cost to battleships, but differed in form and balance of attributes. Battlecruisers typically had thinner armour and a somewhat lighter main gun battery than contemporary battleships, installed on a longer hull with much higher engine power in order to attain greater speeds. The first battlecruisers were designed in the United Kingdom, as a development of the armoured cruiser, at the same time as the dreadnought succeeded the pre-dreadnought battleship. The goal of the design was to outrun any ship with similar armament, and chase down any ship with lesser armament; they were intended to hunt down slower, older armoured cruisers and destroy them with heavy gunfire while avoiding combat with the more powerful but slower battleships. However, as more and more battlecruisers were built, they were increasingly used alongside the better-protected battleships.