Although interest in sustainable fashion continues to grow globally, the transition from fast fashion to sustainable fashion in Indonesia, particularly among environmentally conscious youth, remains relatively limited. The gap between environmental awareness and actual consumption behavior highlights the need to understand the psychological mechanisms driving this behavioral shift. Social media, as a dominant platform shaping public perception, may influence both emotional responses (e.g., environmental guilt) and cognitive evaluations (e.g., awareness of environmental consequences). However, its specific role in facilitating transitions in fashion consumption remains underexplored. Moreover, young men in developing countries, such as Indonesia, remain underrepresented in sustainability research, despite their growing significance as a consumer segment. To address these gaps, this study adopts the Stimulus–Organism–Response (S-O-R) framework to examine how Social Media Influence, as a stimulus, affects two internal states—Environmental Guilt (EG) and Awareness of Environmental Consequences (AEC)—which, in turn, shape consumers’ intention to switch from fast fashion to sustainable fashion. Data were collected using purposive sampling from 142 young Indonesian males (Generation Z and Millennials) and analyzed using Partial Least Squares Structural Equation Modeling (PLS-SEM). The results show that social media influence significantly increases both EG and AEC. Both variables positively influence the intention to switch, with AEC exerting a more potent effect. This study extends the application of the S-O-R framework to the context of sustainable fashion, highlighting the psychological pathways shaped by social media. The study offers practical insights for marketers and policymakers in developing targeted, psychology-informed digital strategies to encourage sustainable consumption among young male consumers.