John Duncan "Jock" Semple (October 26, 1903 – March 10, 1988) was a Scottish-American runner, physical therapist, trainer, and sports official.
In 1967, he attained worldwide notoriety as a race official for the Boston Marathon, when he attempted to tear off the number of Kathrine Switzer, who was officially entered despite a ban on female competitors.
He subsequently oversaw implementation of qualifying times in 1970 and the formal admission of female runners in 1972.