In 1908 and rendered obsolete by the emergence of the dreadnought type of battleships, she once again returned to the reserve, this time with the Home Fleet.

Following the outbreak of the First World War, Jupiter served with the Channel Fleet and then as a guard ship on the River Tyne.

Her propulsion system consisted of two 3-cylinder triple-expansion steam engines powered by eight coal-fired, cylindrical fire-tube Scotch marine boilers.

[2] The ship was armed with a main battery of four BL 12-inch (305 mm) Mk VIII guns in twin-gun turrets, one forward and one aft.

She was also equipped with five 18 in (457 mm) torpedo tubes, four of which were submerged in the ship's hull, with the last in a deck-mounted launcher.

[1] The barbettes for the main battery were protected with 14 in (356 mm) of armor, and the conning tower had the same thickness of steel on the sides.

[6] The following year she was part of a squadron visiting Nauplia and Souda Bay at Crete for combined manoeuvres between the Channel and Mediterranean fleets in September and October.

[9] On 4 February 1908, Jupiter was recommissioned for reserve service in the Portsmouth Division of the new Home Fleet with a nucleus crew.

Her refit completed, Jupiter was commissioned at Birkenhead on 12 August 1915 for service in the Mediterranean Sea on the Suez Canal Patrol.

She was relieved of flagship duty by the troopship RIM Northbrook of the Royal Indian Marine on 9 December 1915 and returned to the Suez Canal Patrol for Mediterranean service.

[4] Jupiter left Egypt on 22 November 1916 and returned to the United Kingdom, where she was paid off at Devonport to provide crews for antisubmarine vessels.

She remained at Devonport until April 1919, in commission as a special service vessel and auxiliary patrol ship until February 1918, when she was again paid off.