A thermoplastic modacrylic resin composition includes a modacrylic resin and a plasticizer. The modacrylic resin includes 15 to 85 wt % of acrylonitrile, 15 to 85 wt % of vinyl chloride and/or vinylidene chloride, and 0 to 20 wt % of another vinyl monomer, and the plasticizer is an organic compound that is compatible with the modacrylic resin and has a boiling point of at least 200° C. A method for manufacturing a thermoplastic modacrylic resin composition includes melt-kneading a powder mixture obtained by mixing a modacrylic resin powder and an organic compound that is compatible with the modacrylic resin and has a boiling point of at least 200° C. Acrylic fibers are composed of a thermoplastic modacrylic resin composition.