Alexander Keith Johnston (28 December 1804 - 9 July 1871) was a Scottish geographer.

He was born at Kirkhill near Edinburgh. After an education at the high school and the university of Edinburgh he was apprenticed to an engraver; and in 1826 joined his brother (afterwards Sir William Johnston , lord provost of Edinburgh) in a printing and engraving business, the well-known cartographical firm of W. and A. K. Johnston .

His interest in geography had early developed, and his first important work was the National Atlas of General Geography , which gained for him in 1843 the appointment of Geographer-Royal for Scotland. Johnston was the first to bring the study of physical geography into competent notice in England. His attention had been called to the subject by Humboldt; and after years of labor he published his magnificent Physical Atlas in 1848, followed by a second and enlarged edition in 1856. This, by means of maps with descriptive letterpress, illustrates the geology, hydrography, meteorology, botany, zoology, and ethnology of the globe. The rest of Johnston's life was devoted to geography, his later years to its educational aspects especially. His services were recognized by the leading scientific societies of Europe and America. He died at Ben Rhydding , Yorkshire, in 1871.

Johnston published a Dictionary of Geography in 1850, with many later editions; The Royal Atlas of Modern Geography, begun in 1855; an atlas of military geography to accompany Alison 's History of Europe in 1848 seq.; and a variety of other atlases and maps for educational or scientific purposes.

His son Alexander Keith Johnston (1844-1879) was also the author of various geographical works and papers; in 1873-1875 he was geographer to a commission for the survey of Paraguay; and he died in Africa while leading the Royal Geographical Society's expedition to Lake Nyasa.