Mieczysław Niedziałkowski (September 19, 1893 in Vilnius - June 21, 1940 in Palmiry) was a Polish politician and writer. He was an activist in the Polish Socialist Party, editor in chief of "Robotnik", and one of the primary activists and cofounders of the Centrolew alliance. He published several works on socialism and Polish politics. He took part in the defence of Warsaw in 1939, organizing the volunteer militias. He was subsequently arrested and interrogated by the Gestapo. He was executed on 21 June 1940 in Palmiry during the German AB-Aktion.