Big John Wrencher (February 12, 1923 – July 15, 1977), also known as One Arm John, was an American blues harmonica player and singer, well known for playing at the Maxwell Street Market in Chicago in the 1960s. He toured Europe in the 1970s.

John Thomas Wrencher was born in Sunflower, Mississippi. He became interested in music as a child and taught himself to play the harmonica at an early age. Beginning in the early 1940s, he worked as an itinerant musician in Tennessee, Missouri, Indiana, and Illinois. By the mid-1940s he had arrived in Chicago and was playing on Maxwell Street and at house parties with Jimmy Rogers, Claude “Blue Smitty” Smith and John Henry Barbee. In the 1950s he moved to Detroit, where he worked with the singer and guitarist Baby Boy Warren and formed his own trio, which performed in the Detroit area and in Clarksdale, Mississippi.