William Borthwick Johnstone, RSA (21 July 1804 – 5 June 1868) was a Scottish landscape and historical painter, art collector, and gallery curator.

Both his father and mother died when he was very young, and he and his younger brother James were placed under the care of Mr. Cunningham, the parish minister of Duns, Berwickshire, where they attended school.

[1] Johnstone's earlier pictures were mainly landscapes and familiar subjects, and these he handled with more elaboration than marked his later productions, which included many historical paintings.

His works show much care and a very genuine feeling for art, but owing to the comparatively late period at which he devoted himself to painting he was never able to acquire complete and easy command over the technique of the craft.

[2] Johnstone was more eminent as a connoisseur and collector than as a painter, and his experience was of great value on the formation of the National Gallery of Scotland in 1858, when he was appointed first principal curator by the Lords of the Treasury.