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.

Carter arrived at County General as a third-year medical student. His career got off to a rocky start when on his first day at County, he nearly vomited in the emergency room after seeing a critically wounded patient and had to be consoled by Chief Resident Dr. Mark Greene (Anthony Edwards).

Carter had a dedicated and compassionate approach with his patients. Initially interested in surgery, he switched to emergency medicine, much to his mentor Dr. Peter Benton's (Eriq La Salle) initial dismay and disappointment. During his surgical residency, Carter laments on the lack of patient connection, specifically the lack of thorough follow-up and care.

In order for Carter to change from his surgical residency to an emergency medicine residency, he agreed to work without pay. He was part of an influential and wealthy family and did not need a salary, enabling County General to take him on despite the lack of funding for an additional position. As a resident, his confidence grew, and he often went out of his way to help patients. During season 9, Carter revealed that his family made its fortune by selling coal at high prices during the Great Depression.

During season 6, Carter and medical student Lucy Knight (Kellie Martin) are stabbed by patient Paul Sobricki (David Krumholtz), a law student suffering from schizophrenia. Knight dies from her injuries while Carter's injuries leave him with lifelong kidney problems. As a result of Carter's chronic battle with pain, survivor guilt, and resistance to getting help, he eventually develops an addiction to narcotics. He begins making mistakes at work. After Abby Lockhart (Maura Tierney) catches him injecting left-over fentanyl from a trauma into his wrist, Carter's colleagues hold an intervention and Dr. Greene demands that he go to an inpatient rehab center for medical doctors in Atlanta or be fired. Although Carter is initially opposed to going, Dr. Benton convinces him and boards the plane with him.

Upon returning from rehab in season 7, Carter makes peace with Chase, and apologizes for his long absence, saying, "I didn't want to admit to the fact that I was just like you." At the end of the season, Kerry Weaver (Laura Innes) rejects Carter's application for Chief Resident because of his history of addiction, stating that his spiral is too recent for him to take on the pressure of that hugely difficult position. However, early in Season 8 a combination of some reflection by Kerry and a gruesome error by Chen on a case that ended with a patient's death leads her to change her mind and offer Carter the CR spot, which he accepts. Carter and the returned Dr. Susan Lewis have a brief and tepid romance that ends on cordial terms when they recognize they're not in love (and Susan recognizes that Carter is romantically interested in Abby Lockhart). Carter has family struggles in Season 8, as his grandmother's health declines, his weak father divorces his icy mother, and his mother arrives in Chicago to a hostile reception from her son. Later in the year, Carter recognizes that the death of Mark Greene means he has to be the ER's leader, and he steps up with a strong and impressive showing when a possible smallpox case leads to the ER being quarantined.

During season 9, Carter and Abby begin sleeping together after they are quarantined in the ER for two weeks during an outbreak of monkeypox. Meanwhile, the health of Carter's grandmother, Millicent (Gamma), continued to decline, and his mother, Eleanor, had difficulty accepting her divorce from Carter's father, Jack. Worse, Abby and Carter continued to disagree over whether or not Abby, a recovering alcoholic, should be drinking at all, even moderately. These personal issues come to a head when Abby's bipolar brother Eric (Tom Everett Scott) reappears the day Gamma dies. Abby giving precedence to her duty as a sister marks the beginning of the end of Carter and Abby's relationship.