Provided are a styrene-based copolymer prepared from a mixture including (A) an aromatic vinyl-based compound, (B) an unsaturated nitrile-based compound, and (C) a silicone-based compound having two or more unsaturated reactive groups to thereby be capable of implementing a uniform and excellent low gloss property with minimal or no deterioration of physical properties such as impact resistance, heat resistance, and the like, and a thermoplastic resin composition including the same.