Knud Maartmann Ringnes (14 April 1875 – 1945) was a Norwegian businessman. He was born in Kristiania, the eldest son of Ellef Ringnes (1842–1929), brewery owner and benefactor, and his wife Kaja, née Maartmann (1851–1933). He was a nephew of Amund Ringnes and maternal grandson of Knud Geelmuyden Fleischer Maartmann, his namesake. He was decorated as a Knight, First Class of the Order of St. Olav. He was also rewarded crosses of the Austrian Order of Franz Joseph and the German Red Cross Decoration.