Results　of　ab　initio　self-consistent-field　(SCF)　and　density　functional　theory　(DFT)　calculations　of　the　gas-phase　structure,　acidity　(free　energy　of　deprotonation,　ΔG○)　and　aromaticity　of　tetraselenosquaric　acid　(3,　4-diselenyl-3-cyclobutene-1,2-diselenone,　H2C4Se4)　are　reported.　The　global　minimum　found　on　the　potential　energy　surface　of　tetraselenosquaric　acid　presents　a　planar　conformation.　The　ZZ　isomer　was　found　to　have　the　lowest　energy　among　the　three　planar　conformers　and　the　ZZ　and　ZE　isomers　are　very　close　in　energy.　The　optimized　geometric　parameters　exhibit　a　bond　length　equalization　relative　to　reference　compounds,　cyclobutanediselenone,　and　cyclobutenediselenol.　The　computed　aromatic　stabilization　energy　(ASE)　by　homodesmotic　reaction　is　-77.4　(MP2(fu)/6-311　+　G*　*//RHF/6-311　+　G*　*)　and　-　54.8　kJ/mol　(B3LYP/6　-　311　+　G*　*//B3LYP/6　-311　+　G*　*).　The　aromaticity　of　tetraselenosquaric　acid　is　indicated　by　the　calculated　diamagnetic　susceptibility　exaltation　(Λ)　-　19.13　(CSGT(IGAIM)　-　RHF/6　-　311　+　G*　*//　RHF/6-311　+　G*　*　and　-32.91(4π·10-6　m3/mol)　(CSGT(I-GAIM)-B3LYP/6　-　311　+　G*　*//B3LYP/6　-　311　+　G*　*).　Thus,　tetraselenosquaric　acid　fulfils　the　geometric,　energetic　and　magnetic　criteria　of　aromaticity.　The　calculated　gas-phase　acidity　is　ΔG○1(298K)　=　1257.7　and　ΔG○2(298K)　=　1617.1　kJ/mol.　Hence,　tetraselenosquaric　acid　is　the　strongest　acid　among　the　three　squaric　acids　(3,4-dihydroxy-3-cyclobutene-1,2-dione,　H2C4O4,　3,4-dithiohydroxy-3-cyclobutene-1,2-dithione,　H2C4S4,　3,4-diselenyl-3-cyclobutene-1,2-diselenone,　H2C4Se4).