The new compounds Rb3Yb7Se12 and CsEr3Se5 have been synthesized through the reaction of elemental rare-earth metal and Se with halide flux at 680 °C. The structures of the compounds were determined by the single-crystal X-ray diffraction method. The structures of CsEr3Se5 and Rb3Yb7Se12 are considerably similar to each other. Rb3Yb7Se12 crystallizes in space group Pnnm of the orthorhombic system with cell dimensions a = 12.385-(4) Å, b = 25.688(7) Å, c = 4.063(1) Å, and Z = 2. CsEr3Se5 crystallizes in space group Pnma of the orthorhombic system with cell dimensions a = 22.059(9) Å, b = 4.095(2) Å, c = 12.155(5) Å, and Z = 4. The structures of these compounds have infinite one-dimensional channels. The channels consist of edge-sharing RESe6 (RE = rare-earth metal) octahedral layers and alkali-metal ions reside in the channels. The RESe6 octahedra in both compounds are distorted with bond lengths varing from 2.752(3) Å to 2,939(2) Å. The result of magnetic susceptibility and resistivity measurement of Rb3Yb7Se12 showed that it is paramegnetic and is an insulator. Structural comparison between Rb3Yb7Se12 and CsEr3Se5 is discussed. A series of additional isostructural compounds Cs3Y7Se12 and Rb3Er7Se12 have been prepared, and structural diagrams of ternary selenides with respect to cationic radius ratios were drawn.