Group V elements are known to show rich allotropic transformation because their semimetallic bonding character can be easily shifted to either metallic or covalent side, for example by changing the applied pressure. Here, we report on our finding that such an allotropic transformation can be induced also by the change in the thickness of the film of Bi on the scale of several atomic layers. Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the Si surface. This pseudocubic {012}-oriented allotrope is stable up to several atomic layers at room temperature. As the thickness increases, the entire volume of the film transforms into a bulk single-crystal (001) phase, due to the increase in the bulk contribution to the cohesion. Based on our ab initio calculations, we propose that the new allotrope consists of black-phosphorus-like puckered-layers stabilized by saturating all the p_z dangling bonds in the film. The resulting film is very flat, compared to the growth of any known metal films, reflecting the inherent two-dimensional (2D) structure of the {012} and the (001) phases.