Many fashion companies are increasing their sustainable offer by introducing into the market new sustainable clothing (e.g., bio-based garments). However, the level of acceptance of these products is still low among consumers due to barriers, such as high prices, lack of product knowledge or availability. To foster sustainable fashion consumption, a clear understanding of consumers’ motivations to purchase this kind of clothing is needed. This paper aims to investigate the determinants of consumers purchasing intentions and behaviors toward bio-based garments. To this aim, a conceptual framework was developed, based on the integration between the “Theory of Planned Behavior” (TPB) and the “Fashion Adoption Theory” (FAT). Sustainable fashion is considered a new trend, according to previous studies; therefore, the sustainable fashion adoption process was studied by reconducting the FAT variables in the TPB model. This study adopts a quantitative methodology. A questionnaire was spread among Italian consumers. The linear regression technique will be used for data analysis. The contributions of this study are manifold. From a theoretical point of view, the adoption of different behavioral theories and their modification using innovative and traditional variables in different combinations. From a managerial point of view, (1) insights for developing marketing strategies for sustainable garments and specific consumer segments; and (2) information to evaluate how to best use digital channels to promote its sustainable offer. Concerning policy-related implications, this research will help define categories of target consumers on which to focus awareness campaigns.