We present a novel magnetic semiconductor, Cr2.37Ga3Se8, synthesized by partially replacing magnetic Cr3+ in antiferromagnetic Cr5+delta Se8 with nonmagnetic Ga3+. The crystal structure of Cr2.37Ga3Se8, was determined by both powder and single-crystal X-ray diffraction. The title compound crystallizes in a monoclinic structure with space group C2/m (No. 12). In Cr2.37Ga3Se8, the Cr atoms are surrounded by 6 Se atoms and form filled octahedral clusters, while Ga atoms are centered in the Se-4, tetrahedral clusters. The two kinds of clusters pack alternatingly along the c-axis, which results in a quasi-two-dimensional layered structure. The magnetization (M) measurement shows the development of short-range ferromagnetic coupling below the Curie Weiss (CW) temperature theta(cw) similar to 92 K, evidenced by the nonlinear field dependence of M. However, the magnetic susceptibility exhibits a peak at low fields at similar to 18 K, indicating the existence of an antiferromagnetic interaction as well. Electronic structure calculations using the WIEN2k program in the local spin density approximation indicate that the magnetism arises exclusively from local moments of the Cr atoms. The electrical resistivity measurement of the Cr2.37Ga3Se8 sample confirms that this material is a semiconductor with the band gap similar to 0.26 eV. Meanwhile, the experimental band gap (,similar to 0.26 eV) is close to the theoretical prediction using the WIEN2k program (similar to 0.35 eV).