Aside from amorphous phases, only a single crystalline tin borate had been synthesised so far under high pressure. In this work, we present the very first crystalline ternary tin borate Sn3B4O9 synthesised under ambient pressure by decomposition of Sn3[B3O7]F above 500 °C. The crystal structure of Sn3B4O9 (P21/c, Z = 4, a = 768.07(3) pm, b = 1206.78(4) pm, c = 924.96(3) pm, β = 101.847(1)°, 10 550 data, 147 parameters, R1 = 0.028) determined by single-crystal X-ray diffraction comprises open layered borate polyanions with Sn(ii) ions in-between showing the presence of a stereochemically active lone pair. Sn3B4O9 was further characterized by DFT calculations and vibrational spectroscopy. Its optical band gap was calculated to approx. 3.5(1) eV. Tin borate was gained by thermal decomposition of Sn[B2O3F2] via a further new tin borate fluoride Sn2[B7O12]F (C2/c, Z = 8, a = 1037.99(2) pm, b = 859.78(2) pm, c = 2370.71(8) pm, β = 93.5650(10)°, 2674 data, 199 parameters, R1 = 0.045).