Eugen Dido Kvaternik (1910, Zagreb - 1962) was a Croatian Ustaše General-Lieutenant and the Chief of the Ustaška nadzorna služba (UNS), Internal Security Service in the Independent State of Croatia, a Nazi puppet state during World War II. He was considered at the time the second most important person in Croatia, after Ante Pavelić.
Kvaternik instituted a regime of terror against Croatian Serbs, Jews, Gypsies and other "enemies of the State", and is believed to bear direct and primary responsibility for the atrocities committed by the Ustaše against those groups. In 1943, after a falling-out with Pavelić, the leader of the Independent State of Croatia, he and his father Slavko Kvaternik, who was the Croatian Minister of War, went into exile in Slovakia, and after the war fled to Argentina with the help of the Catholic Church.
From Argentina, Kvaternik continued in his subversive activities against Josip Broz Tito. He reorganized the Ustasha supporters and continued to publish actively, never expressing any regret for his past deeds. Yugoslavia's multiple extradition requests were all turned down, and Kvaternik was never tried.
Eugen "Dido" Kvaternik and his two daughters died in a car crash in Río Cuarto, Argentina in 1962.