John Russell Carnahan (; born July 10, 1958) is an American politician from the state of Missouri. A member of the Democratic Party, he served as the U.S. representative for Missouri's 3rd congressional district from 2005 to 2013.

At the time, the district included the southern third of the city of St. Louis (known as South City) and most of the southern St. Louis suburbs including most of Jefferson County and all of Ste. Genevieve County. Some cities located in the district include: Webster Groves, Mehlville, Affton, and Oakville, and the southern suburbs of Arnold, Herculaneum, Pevely, Crystal City, Barnhart, Imperial, and Festus, as well as Ste. Genevieve in the neighboring Ste. Genevieve County.

Dramatic losses in population in St. Louis in the 2010 Census contributed to Missouri losing a Congressional seat effective 2013. In the re-mapping of the state's congressional districts, Carnahan's district was dismantled. The bulk of the district, including Carnahan's home, was drawn into Missouri's 1st congressional district. The move placed Carnahan and William Lacy Clay Jr. in the same district; Carnahan lost the primary to Clay for the seat on August 7, 2012.As of 2023, he is the Chair of the Missouri Democratic Party.