The fast fashion industry is considered to be the second most polluting industry in the world, facing many challenges regarding environmental sustainability. Due to the rising interest and awareness in environmental sustainability, this study was conducted to discover how fast fashion companies make decisions regarding environmental sustainability and how these decisions are affected by the fast fashion business model. A qualitative case study approach was employed, focusing on the Swedish activewear brand Stronger as the case study company. An in-depth interview was conducted with the company to gain valuable insights into the relationship between the fast fashion business model and firms decisions related to environmental sustainability. The analysis of the data revealed key themes and patterns, highlighting the challenges, opportunities, and potential strategies for firms aiming to incorporate environmental sustainability into their operations within the fast fashion industry. The findings indicated that the fast fashion business model contradicts the principles of environmental sustainability, requiring significant modifications to align with sustainable practices. Based on the findings and analysis, it was also concluded that the firms’ decisions are influenced by the fast fashion business model, however it is possible to make decisions that prioritize environmental sustainability despite the constraints of the business model.