K39 may refer to: 15 cm Kanone 39, a German heavy gun used in the Second World War Junkers K 39, a German prototype reconnaissance-bomber built 1926 K-39 (Kansas highway), a state highway in Kansas Piano Concerto No. 2 (Mozart), K. 39, by Wolfgang Amadeus Mozart K-39, a 1964 studio album by surf rock band The Challengers Potassium-39 (K-39 or 39K), an isotope of potassium