Johannes Nicolaus Brønsted (February 22, 1879-December 17, 1947) was a Danish physical chemist.
He received a degree in chemical engineering in 1899 and his Ph. D. in 1908 from the University of Copenhagen. He was immediately appointed professor of inorganic and physical chemistry at Copenhagen.
In 1906 he published his first of many papers on affinity. In 1923 he introduced the protonic theory of acid-base reactions, simultaneously with the English chemist Thomas Martin Lowry. The same year, the electronic theory was proposed by Gilbert N. Lewis, but both theories are commonly used.
He became known as an authority on catalysis by acids and bases.
In World War II he opposed the Nazis, and in consequence he was elected to the Danish parliament in 1947, but could not take his seat because of illness. He died shortly after election.