We present bidirectional polymorphic conversion of L-phenylalanine by focused femtosecond laser irradiation. When the femtosecond laser beam is focused at an air/solution interface of its unsaturated solution, the plate-like anhydrous crystals are generated from the laser focus. This crystal nucleation is realized by local concentration increase achieved by femtosecond laser trapping, which is the first demonstration of femtosecond laser trapping-induced crystallization. Furthermore, the whisker-like monohydrate crystals are produced by continuing the femtosecond laser irradiation on the surface of the plate-like crystals. On the contrary, when a focused femtosecond laser irradiation is applied to the whisker-like monohydrate crystals, the plate-like crystals are formed, meaning that an unstable phase is successfully produced from a stable phase. The dynamics and mechanism of the bidirectional polymorphic conversion are explained from the viewpoint of the nucleation rate theory considering solution concentration around the surface of laser-induced cavitation bubbles.