Alexander Keith Johnston (1844, November 11 in Edinburgh – 1879, June 28 in Tanzania) was a Scottish explorer and geographer.
He was the son of published geographer Alexander Keith Johnston (1804-1871) and Mary Grey.
From 1873 to 1875 he was geographer to a commission for the survey of Paraguay. He led a Royal Geographical Society expedition to Lake Nyasa and Tanganyika. Johnston died in Beho Beho in South Africa after 6 months of this expedition from malaria and dysentry.
He is remembered on his father's grave in Grange Cemetery in Edinburgh.