HMS Mercury was one of two Iris-class despatch vessels, later redesignated as second class cruiser built for the Royal Navy during the 1870s.

The ships displaced 3,730 long tons (3,790 t) at normal load[1] and were the first British warships with an all-steel hull.

Mercury reached a maximum speed of 18.57 knots (34.39 km/h; 21.37 mph) from 7,735 ihp (5,768 kW), making her the fastest warship in the world.

[3] The ship carried enough coal to steam 4,950 nautical miles (9,170 km; 5,700 mi) at 10 knots (19 km/h; 12 mph).

[1] Originally equipped with a light barque rig, her sails were soon removed and the class became the first "mastless cruisers".