John Jenner Weir, FLS, FZS (9 August 1822 – 23 March 1894) was an English amateur entomologist, ornithologist and British civil servant. He is best known today for being one of the naturalists who corresponded with and provided important data to both Charles Darwin and Alfred Russel Wallace. He played a particularly important role in the formulation of Wallace’s theory of aposematism, providing the first experimental evidence for the effectiveness of warning coloration in caterpillars. Weir was one of the three judges at the first cat show, in Great Britain.