The influence of the solvent content on the solution-mediated phase transformation of sulfadiazine (SD) N-methyl pyrrolidone (NMP) solvate into SD was investigated for process development. The solubility results of the NMP solvate and SD in NMP solvent/water mixtures supported by the analysis of solvent-solvent interactions between NMP solvent molecules were used to understand and optimize the transformation of the NMP solvate into SD. Needle-on-rhombic-like aggregated crystals and rod-like single crystals of SD were obtained and reported for the first time. By combining Raman spectroscopy, focused-beam reflectance measurements, and scanning electron microscopy, the phase transformation of NMP solvate into SD was tracked. Higher temperatures and stirring rates accelerate the transformation process and have a negative influence on the final SD product quality.