Tadeusz Zygfryd Kassern (19 March 1904 – 2 May 1957) was a Polish composer.
Born in Lvov, he studied at the conservatory of the Polish Music Society in Lviv and later at the conservatory in Poznań. In 1931 he escaped from Nazism to Paris and later to the United States. Kassern had been serving as a cultural attaché in New York to the Polish government. In 1948 when Kassern learned of the communist takeover in Poland, he resigned his position, renounced his Polish citizenship and applied for US asylum. Kassern's application for asylum was denied due to missing a deadline set for asylum seekers; depressed, he attempted suicide unsuccessfully. Kassern died of cancer in 1957.
Among Kassern's well known students was the Italian-born American composer Dalmazio Santini (1923-2001).