Crystals of a new polymorph (α-form) of the known semi-organic nonlinear optical (NLO) compound bismuth thiourea chloride (Bi[CS(NH₂)₂]₃Cl₃, BTC) have been successfully grown from hydrochloric acid solutions of thiourea and bismuth chloride. Single crystal X-ray diffraction reveals that α-BTC crystallizes in a monoclinic space group Cc with unit cell dimensions of a = 8.503(13) Å, b = 11.983(17) Å, c = 15.39(2) Å, β = 90.509(18)° and Z = 4. It is found that in a wet environment, the structure of α-BTC changes from Cc to P3 with colour changing slowly from yellow to orange. Both optical transmission and UV diffuse reflection indicate that α-BTC has a transmittance cutoff at 500 nm about 60 nm blue shifted from its counterpart (β-BTC) in P3 space group. Furthermore, thermogravimetric (TG) and differential thermal analysis (DTA) show that α-BTC decomposes above 200 °C and an intermediate form also exists around 140 °C. Preliminary structure study shows that the intermediate form (γ-BTC) belongs to space group P1 with cell parameters of a = 7.372(11) Å, b = 7.470(9) Å, c = 15.34(2) Å, α = 99.76(5)°, β = 94.32(6)°, γ = 110.35(4)° and Z = 2. Together with a recent reported water containing BTC, it is interesting to find that a single compound can exist in four polymorphs and all possessing noncentrosymmetric structures and nonlinear optical properties.