Treat Baldwin Johnson (1875-1947) was an American chemist, born at Bethany, Connecticut.
He graduated at Yale in 1898, where he also received his Ph.D. in 1901. He became an instructor of chemistry at the Sheffield Scientific School of Yale and in 1908 was advanced to the assistant professorship of organic chemistry of which branch he became full professor in 1915. He published papers on organic synthesis as applied to therapeutic substances, on phenanthrene and its relation to morphine, an account of new local anæsthetics, histamin, tyramin, and cyclic polypeptids. In 1918, he received the Nichols Medal of the American Chemical Society.
Johnson was the editor of the owner's manual for A. C. Gilbert Company's chemistry sets. He, together with R.D. Coghill, was the first to discover the existence of 5-Methylcytosine in nature, from tuberculinic acid, a nucleotide of Mycobacterium tuberculosis.