Maksymilian Horwitz (pseudonym: Henryk Walecki; 1877-1937) was a leader and theoretician of the Polish communist movement.

Maksymilian Horwitz was born to a Jewish family in Warsaw, as a son of Gustaw Hortwiz and Julia Kleinmann.

He was a member of the Polish Socialist Party - Left (PPS-Lewica) from 1906 and the Communist Party of Poland (KPP) from 1918 and sat on its Central Committee (1918–20, 1923–24) and politburo (1923–24). Starting from 1921 he was a senior officer of the Communist International, working in Spain, Belgium and Greece. During The Great Purge, He was arrested by the NKVD on 21 June 1937 and executed on 20 September 1937.

Hortwitz's niece, Hanna Mortkowicz-Olczakowa and her daughter, Joanna Olczak-Ronikier are writers.