Recep Tayyip Erdogan hit out after German authorities cancelled rallies designed to woo ethnic Turkish voters in Germany ahead of a key referendum.
"Your practices are not different from the Nazi practices of the past," Mr Erdogan said.
German Justice Minister Heiko Maas said the comments were "absurd, disgraceful and outlandish".
About 1.4m Turks in Germany can vote in the April referendum, which could give Mr Erdogan sweeping new powers.
