James Finlay Weir Johnston, FRS (13 September 1796 – 18 September 1855) was a Scottish agricultural chemist.
Born in Paisley, Renfrewshire, Johnston was educated at University of Glasgow, acquired a fortune by his marriage in 1830, and devoted himself to studying chemistry. He visited the chemist J. J. Berzelius in Sweden and was a co-founder of the British Association for the Advancement of Science. After some years he was chosen as a lecturer in Durham University, but he resided in Edinburgh, and wrote his Catechism of Agricultural Chemistry (1844), since translated into most European languages, and his Chemistry of Common Life (1853–1855). From 1847, his assistant was Augustus Voelcker who also lectured in agricultural chemistry at Durham University. Johnston died in Durham on 18 September 1855.
Johnston formed a grammar school in Durham, which later merged with other local schools, such as Brandon and Bowburn, to form a single comprehensive school for the area, named Durham Johnston Comprehensive School, preserving James Finlay Weir Johnston's name.