HMS Diadem was the lead ship of the Diadem-class of protected cruiser in the Royal Navy.

[1] She served in the Easter Division of the Channel Squadron under the command of Captain Harry Seawell Niblett, and was briefly docked at Chatham in January 1900 to make good defects.

[2] In March 1901 Diadem was one of two cruisers to escort HMS Ophir, commissioned as royal yacht for the World tour of the Duke and Duchess of Cornwall and York (later King George V and Queen Mary), from Spithead to Gibraltar,[3] and in September the same year she again escorted the royal yacht from St Vincent to Halifax, Nova Scotia.

[4] She was paid off at Chatham on 11 February 1902, and in May transported to Fairfield Shipbuilding and Engineering Company in Glasgow for repairs to her hull and machinery.

She was returned to being a stokers' training ship in January 1918, and survived the war to be sold to Thos.