Rabbi Shmuel Dovid Ungar (23 November 1885 – 9 February 1945), also known as Rabbi Samuel David Ungar, was the rabbi of the Slovakian city of Nitra and dean of the last surviving yeshiva in occupied Europe during World War II.
He was the father-in-law of Rabbi Chaim Michael Dov Weissmandl, and played a minor role in the Bratislava Working Group's efforts to save Slovak Jews from the Holocaust.