John Howard Northrop (July 5, 1891 - May 27, 1987) was an American biochemist who won the Nobel Prize in Chemistry in 1946 (with James Batcheller Sumner and Wendell Meredith Stanley) for purifying and crystallizing certain enzymes.

Northrop was born in Yonkers, New York and educated at Columbia University, where he earned his PhD in chemistry in 1915.
During World War I, he conducted research for the U.S. Army Chemical Warfare Service on the production of acetone and ethanol through fermentation.
In 1929 he isolated and crystallized the gastric enzyme pepsin and determined that it was a protein and in 1938 he isolated and crystallized the first bacteriophage, and determined that it was a nucleoprotein. Northrop also isolated and crystallized pepsinogen (the precursor to pepsin), trypsin, chymotrypsin, and carboxypeptidase.
His 1939 book Crystalline Enzymes was an important text. Northrop was employed by the Rockefeller Institute for Medical Research in New York City from 1916 to 1961, at which time he retired. Northrop died in Wickenberg, Arizona.