The Colossus class of two battleships of the British Royal Navy were among the first dreadnoughts following the original Dreadnought. Originally intended to be called the Neptune class, the two ships had thicker armour and other differences from Neptune, and so Neptune is traditionally left as its own class. They were the last 12-inch gunned dreadnought battleships for the Royal Navy, and were followed by the first "super-dreadnoughts", the Orion_class_battleships.

In designing this class, Their Lordships finally took note of the vulnerability of British capital ships in view of the general move to 12-inch guns in the German navy. Thus the main belt reverted to the 11-inch thickness of the Dreadnought, with the unfortunate corollary of necessitating a reduction in internal protection to compensate.

Machinery was for the first time divided into 3 compartments instead of 2. This was thought to provide resistance to flooding to balance the loss of screening bulkheads. Machinery was otherwise identical to that fitted previously, except apparently the new internal arrangements allowed the center engine room to operate independently during cruising conditions to conserve fuel.

Main armament remained the same as the Neptune Class, except the stagger of the wing turrets was reduced to conserve deck space. This enabled the forward superstructure to be lengthened and the distribution of the secondary armament to be improved.

To save 50 tons of topweight, the mainmast was suppressed. Besides the weight, the mainmast was considered to be of limited value. Meanwhile the Dreadnought's design flaw of locating the foremast just behind the fore funnel was inexplicably repeated. In the Colossus class, this situation was aggravated by the far larger power of the installation served by the funnel.