Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3, 1975, and designated S/1975 J 1.
Prvými objaviteľmi boli 30. septembra 1975 Charles T. Kowal a Elizabeth Roemer a svoj objav oznámili 3. októbra 1975, kedy dostal označenie S/1975 J 1.