21st Brigade (21st Bde) was an infantry formation of the British Army first organised in the Second Boer War, when it took part in Ian Hamilton's March from Bloemfontein to Pretoria.

British Army brigades had traditionally been ad hoc formations known by the name of their commander or numbered as an integral part of a division.

[5][6][7] His new brigade comprised three Regular infantry battalions together with the famous City Imperial Volunteers (CIV), recently arrived from Britain.

[3][8][9] The composition of 21st Bde in April–June 1900 was as follows:[3][10][11] 21st Brigade began operations as part of Lord Roberts' force and was then assigned to Lieutenant-General Ian Hamilton's Winburg Column.

Thereafter its units continued to participate individually in the efforts to catch Christiaan de Wet and the other Boer forces still at large – Bruce Hamilton still had the Sussex and Cameron Highlanders with him during the autumn campaign – but the brigade organisation had disappeared.