A novel peroxopolyoxomolybdate compound, [N(CH3)4]3 [H(SO4)2 (MoO2(O2))3] · 3H2O, has been obtained from a strongly acidic aqueous solution of a molybdate-sulfate system as the monoclinic crystal of P2/n, a = 13.497(3), b = 15.231(4), c = 32.603(2) Å, β = 103.113(9)°, and V = 6577(2) Å3. The final R-value was 0.0986 for 395 parameters using 7162 reflections. Each of the two crystallographically independent polyanions contains three molybdenum atoms with five O2-, one O22-, and two sulfate groups, one of which is a tripod ligand apical to the molybdenum plane, and the other is a bipod ligand equatorial to the Mo plane.