The Formidable class of battleships were a three-ship class of pre-dreadnoughts designed by Sir William White and built for the Royal Navy in the late 1890s. The class comprised Formidable, Irresistible, and Implacable. They were armed with a battery of four 12-inch (305 mm) guns, they had top speed of 18 knots, and they marked the adoption of Krupp armour in British battleship designs. The class formed the basis for the nearly identical London class of five ships, and those ships are sometimes included in the Formidable class. Formidable, Irresistible, and Implacable were built between 1898 and 1901 at the Portsmouth, Chatham, and Devonport Dockyards, respectively.

Pre-dreadnought battleships were sea-going battleships built from the mid- to late- 1880s to the early 1900s. Their designs were conceived before the appearance of HMS Dreadnought in 1906 and their classification as "pre-dreadnought" is retrospectively applied. In their day, they were simply known as "battleships" or else more rank-specific terms such as "first-class battleship" and so forth. The pre-dreadnought battleships were the pre-eminent warships of their time and replaced the ironclad battleships of the 1870s and 1880s.