Robert McCarty Knapp (April 21, 1831 – June 24, 1889) was a U.S. Representative from Illinois, brother of Anthony Lausett Knapp. Born in New York City, Knapp moved with his parents to Jerseyville, Illinois, in 1839.He attended the common schools and the Kentucky Military Institute in Frankfort, Kentucky.He studied law.He was admitted to the bar in 1855 and commenced practice in Jerseyville.He served as member of the Illinois House of Representatives in 1867.He served as mayor of Jerseyville 1871-1876.