Johann Georg Rapp was born on November 1, 1757, to Rosine Berger and Hans Adam Rapp (1720–71) in the village of Iptingen, northwest of Stuttgart in the Duchy of Württemberg. Rapp was the second child and oldest son of the family. His brother, Adam (born on March 9, 1762), and three sisters, Marie Dorothea (born October 11, 1756), Elise Dorothea (born August 7, 1760), and Maria Barbara (born October 21, 1765) later followed him to America; however, Adam died at sea.