A new iron selenide, K2FeGe3Se8, has been obtained by spontaneous crystallization. It adopts a new group P2(1). In the structure, FeSe4 and GeSe4 tetrahedra are : structure type in the noncentrosymmetric monoclinic space connected alternately via corner-sharing to form one-dimensional (1D) (1)(infinity)[FeGeSe6](6-) chains along the a-direction. These chains are further linked by sharing Ge2Se6 units to generate two-dimensional (2D) (2)(infinity)[FeGe3Se8](2-) layers stacked parallel to the ac-plane and separated by K+ cations. Deduced from temperature-dependent susceptibility measurement and specific heat measurement under different magnetic fields, K2FeGe3Se8 exhibits an antiferromagnetic transition at similar to 10 K. Furthermore, the magnetic property shows anisotropy between directions parallel and perpendicular to the plane of (2)(infinity)[FeGe3Se8](2-) layer. The diffuse reflectance spectra measurement indicates that the band gap of K2FeGe3Se8 is similar to 1.95(2) eV, consistent with the calculated values of 1.80 and 1.53 eV in the spin-up and spin-down directions, respectively. Based on electronic structure calculation, the spin of the Fe2+ cation is 1.85 (h) over bar, which is comparable to the experimental value.