Ternary selenometallates, Li₅MSe₄ (M = Al(I) and Ga(II)), have been synthesized for the first time through high temperature solid-state reactions combining elements and Li₂Se in stoichiometric compositions. Li₅MSe₄ crystallizes in the P2₁/m space group, forming a pseudo-2D layer type structure with edge sharing LiSe₄ and MSe₄ tetrahedra along the a-axis. These layers are interleaved by octahedrally coordinated Li ions located in the interlayer space. AC impedance spectroscopy measurements yield room temperature ionic conductivities of 0.60 × 10⁻⁷ and 0.58 × 10⁻⁷ S cm⁻¹ with calculated activation energies of 0.51 and 0.48 eV for I and II, respectively. An aliovalent substitution of Sn⁴⁺ in Li₅MSe₄ yields compositions of Li_4.66Al_0.82Sn_0.22Se₄ (III) and Li_4.37Ga_0.89Sn_0.24Se₄ (IV), which crystallize in the P2₁/m and Pm1 space groups, respectively. Sn-doped samples show an ∼5-fold increase in ionic conductivity, 3.37 × 10⁻⁷ S cm⁻¹ and 2.4 × 10⁻⁷ S cm⁻¹ with activation energies of 0.54 and 0.28 eV, respectively, for III and IV. The optical band gap values of the compounds are 3.65 and 3.2 eV for I and II, respectively, as measured by diffuse reflectance spectroscopy. Density functional theory (DFT) calculations predicted a major contribution from the Se 4p-states in forming the top of the valence band and strongly hybridized the Se 4p and ns orbitals of Al and Ga in forming the bottom of the conduction band with almost no contribution from the Li s-states near the Fermi level indicating their ionic interactions with the ligand.