Alexander Keith Johnston FRSE FRGS FGS FEGS LLD (28 December 1804 – 9 July 1871) was a Scottish geographer and cartographer.
He was born at Kirkhill near Penicuik, south of Edinburgh. He was the son of Andrew Johnston and Isabel Keith. His brother was Thomas Brumby Johnston FRSE.
After an education at the High School and the University of Edinburgh he was apprenticed to the Edinburgh engraver and mapmaker, James Kirkwood. In 1826, he joined his brother William (who would become Sir William Johnston, Lord Provost of Edinburgh) in a printing and engraving business, forming the well-known cartographical firm of W. and A. K. Johnston with offices based at 4 St Andrew Square in Edinburgh's New Town (demolished 2016) and their printworks based at Edina Works, off Easter Road. The firm used the clan motto of "Ready Aye Ready" as their logo. Early hikes in the West Highlands had led Johnston to despair at the accuracy of maps, and inspired a deire to rectify this. The rest of Johnston's life was devoted to geography, his later years to its educational aspects especially. His services were recognised by the leading scientific societies of Europe and America.
In October 1849, he was elected a Fellow of the Royal Society of Edinburgh, his proposer being Robert Chambers. In 1862 he was a founding member of the Meteorological Society of Scotland. He was awarded an honorary doctorate (LLD) in 1865.
His Edinburgh address in later years was 16 Grosvenor Crescent in the affluent West End.
He died at Ben Rhydding, Yorkshire, in 1871. He is buried in the north-west section of Grange Cemetery in Edinburgh.