Filemon Castelar Lagman (March 17, 1953 – February 6, 2001), popularly known as Ka Popoy, was a revolutionary socialist and workers' leader in the Philippines. He shares the ideology of Marxism-Leninism. He split with the Communist Party of the Philippines in 1991 to form Bukluran ng Manggagawang Pilipino (BMP) and the multi-sectoral group Sanlakas.