Solvent-mediated polymorphic transformation (SMPT) of the alpha-form into the beta-form of taltirelin (TTL) in seeded crystallization was investigated. The kinetics of polymorph transformation of the TTL was found to be a three-dimensional growth of nuclei mechanism by the Avrami-Erofeev equation. The transformation from the alpha-form into the beta-form was also studied by crystallization adding alpha-form seeds in solution. The effects of seed loading amount, temperature, supersaturation, and transformation time on polymorph transformation were evaluated. Seeded crystallization using alpha-form crystals as a seed nucleated the alpha-form TTL and then transformed the alpha-form into the beta-form. The transformation rate was controlled by adjusting supersaturation.