James Henry Duncan (September 25, 1887 – January 21, 1955) was an American discus thrower who won a bronze medal at the 1912 Summer Olympics.

On May 26, 1912, he hurled the discus with his right hand 156 feet 1¾ inches (47.59) at the Irish American Athletic Club's track & field, Celtic Park in Queens, New York.

On the same day, he hurled the discus 96 feet 7.5 inches, with his left hand, breaking the world's record for right and left hands combined with a distance of 252 feet 8 and 7/8 inches.

After his discharge from the U.S. Army, he stayed in France, married a French woman and opened a gymnasium in Paris.

"[5] Duncan was critically injured in 1932, when in an apparent suicide attempt, he shot himself three times, with two bullets lodging in his abdomen.