Robert John Thornton was an English physician and botanical writer, noted for "A New Illustration of the Sexual System of Carolus Von Linnæus" and "The British Flora" of 1812.
He was the son of Bonnell Thornton and studied at Trinity College, Cambridge.
Inspired by Thomas Martyn"s lectures on botany and the work of Linnaeus he switched from the church to medicine. He worked at Guy"s Hospital in London, where he later lectured in medical botany. After spending some time abroad, he settled and practised in London.