Mieczysław Jastrun born Mojsze Agatstein (29 October 1903 – 22 February 1983) was a Polish poet and essayist of Jewish origin.

At the onset of World War II, Jastrun took refuge in Lviv, then occupied by the Soviet Union, where he worked as a teacher.

After Nazi Germany occupied Lviv in 1941, he relocated to Warsaw, took the name Jan Klonowicz and worked as an underground teacher.

He joined the communist Polish United Workers' Party (PZPR), and in the post-war era wrote several books dealing with the Nazi occupation and the Holocaust in the style of socialist realism.

Jastrun also wrote biographies of Jan Kochanowski, Adam Mickiewicz and Juliusz Słowacki, translated works by poets such as Lorca, Pushkin, Hölderlin and Rilke into Polish, and taught modern poetry at the University of Warsaw.