John Truman Carter III, M.D. is a fictional character from the NBC television series ER. He was portrayed by Noah Wyle and appeared as one of the series' principal characters from the pilot episode until the eleventh-season finale. Carter's career path is one of the main story arcs of the series, beginning as a third-year medical student, becoming a resident, first in surgery and then in emergency medicine, before being promoted to an attending physician.

In the twelfth season, Wyle made guest appearances in four episodes, from "Quintessence of Dust" to "There Are No Angels Here". During the fifteenth season, Wyle again reprised the role for five additional episodes, first returning in "The Beginning of the End" and ending with the series finale. Carter is one of the few characters that appears in the pilot of the show and finale of the entire series.