For the Canadian politician, see John Duncan (Canadian politician).

John James Duncan, Jr. usually known as Jimmy Duncan (born July 21, 1947) is a Tennessee Republican politician, representing the state's 2nd Congressional District (map) , which centers around Knoxville.

Duncan was born in Lebanon, Tennessee. He graduated from the University of Tennessee in 1969 with a Bachelor of Science degree and subsequently received a law degree from George Washington University in Washington, D.C. in 1973 and was admitted to the bar that same year. He was an attorney in private practice until he became a state court judge in Knox County, Tennessee, where he served from 1981 to 1988. He also served in the Army National Guard from 1970 to 1987.

He was first elected to Congress in 1988, in a special election to succeed his father, John Duncan, Sr. and elected to the seat in his own right the same day. He has been reelected every two years since then from a district that has been held continously by Republicans since 1857. Like his father, he has never faced a serious or well-funded challenge for reelection. From 1994 through 2000, Duncan was reelected without major-party opposition.

Duncan is very conservative even by Republican standards. He was one of the few Republicans to oppose and vote against the U.S. invasion of Iraq, doing so on grounds of opposition to unnecessary foreign involvement.