Rosenberg was born on 12 January 1893 in Tallinn in the Russian Empire, the capital of modern Estonia, to a family of Baltic Germans. His father, Waldemar Wilhelm Rosenberg, was a wealthy merchant from Latvia, and his mother, Elfriede (née Siré), was a Teacher of French language in Tallinn. The Hungarian-Jewish Journalist Franz Szell, who was apparently residing in Tilsit, Lithuania, spent a year researching in Latvian and Estonian archives before publishing in 1936 an open letter, with copies to Hermann Göring, Joseph Goebbels, German foreign minister Konstantin von Neurath, and others, accusing Rosenberg of having "no drop of German blood" flowing in his veins. Szell wrote that among Rosenberg's ancestors were only "Latvians, Jews, Mongols, and French." As a result of his open letter, Szell was deported by Lithuanian authorities on September 15, 1936. His claims were repeated in the 15 September 1937 issue of the Vatican newspaper L'Osservatore Romano.