Radek was born in Lemberg, Austria-Hungary (now Lvov in Ukraine), as Karol Sobelsohn, to a Jewish Litvak family; his father, Bernhard, worked in the post office and died whilst Karl was young.
Radek lahir di Lemberg , Austria-Hungaria (sekarang Lviv di Ukraina ), sebagai Karol Sobelsohn, dari keluarga Yahudi ; ayahnya, Bernhard, bekerja di kantor pos dan meninggal ketika Karl masih muda.