According to Hitler's lawyer, Hans Frank, Hitler's grandfather was a young Jewish man named Leopold Frankenberger, whose son to Maria Schicklgruber later became Alois Hitler, Adolf's father.
Scholars today discount Frank's story as impossible and say Adolf's real father was most likely Johann Georg Hiedler, who later married Maria and adopted her son Alois.
However, Hitler himself was concerned enough in 1931 about possible Jewish ancestry to order an "investigation" which, not surprisingly, found no such evidence.
So Hitler's alleged Jewish ancestry may be an anti-Nazi or anti-Semitic political myth, but if Hitler himself was worried about it enough to take action, I'd not dismiss it 100%.
Sorry for my mistype -- Alois was Adolf's father, Johann Georg Hiedler is now said to be Alois' real father, not the supposed Leopold Frankenberger.