Ferromagnetism in Ⅰ-Ⅲ-Ⅵ2 chalcopyrite semiconductors is predicted to arise from holes provided by Mn doping in the illustrative case of CuGaSe2. The first-princi plescalculations within the generalized gradientapproximation to density functional theory demonstrate that Mn substitutes for Ga sites in CuGaSe2 with a formation energy that is～0.25 eV lower than that for Cu sites. Ferromagnetic CuGa1-xMnxSe2 is found to be a half-metallic material with a magnetic moment of 4 µB per Mn for x≥0.25, and its estimated Curie temperature is more than 110 K. We suggest that higher Curie temperatures may be achieved for this new class of ferromagnetic semiconductors based on Mn-doped Ⅰ-Ⅲ-Ⅵ2 chalcopyrites by employing other materials with a smaller lattice constant such as CuGaSe2 and CuAlSe2.