Ferroelectric semiconductors, combining semiconduction, spontaneous polarization, and photoinduced excitation, show great promise to enhance the performance of solar cells, pressure sensors, and photodetectors. Particularly, organic-inorganic lead halide perovskite ferroelectrics have been explored for their prominent carrier transport properties and structural tunability. However, a high concentration of toxic Pb is a stumbling block for their further application. Here, we present a lead-free hybrid perovskite semiconductor, (C₄H₉NH₃)₂(NH₃CH₃)₂Sn₃Br₁₀ (1), which exhibits a large spontaneous polarization of 11.76 mu Ccm(2) at room temperature. Significantly, 1 presents a spontaneous polar ordering transition, similar to the better-known perovskite ferroelectrics, and exhibits ferroelectric phase transition behaviors. To our best knowledge, 1 is the first example of a Sn-based hybrid perovskite semiconductor featuring ferroelectric performance. Mechanistic studies reveal that such ferroelectricity can be attributable to the synergistic effects of ordering of organic cations and stereochemically active lone-pair electrons inducing distortion of inorganic octahedra. This work provides an effective way to explore "green" ferroelectric semiconductors with potentially enhanced energy conversion efficiency.