In the context of the fast fashion industry, this study examined the efficacy of sustainable collections in achieving corporate legitimacy. We examine the ethical branding and organizational legitimacy relationship in the context of the fast fashion industry. We conducted an online experiment with the use of Amazon MTurk in the Unites States. The data were collected from female fast fashion users (N = 389). The data analysis revealed that fast fashion retailers' sustainable collections had a positive impact on corporate legitimacy, CSR perception, brand trust, and purchase intention. This study also uncovered that the company's sustainable collections increased consumers' attributions to altruistic motives and consumers' altruistic attributions had a positive influence on corporate legitimacy, CSR perception, brand trust, and purchase intention. Furthermore, this study found the mediating role of altruistic attributions in explaining the impact of sustainable collections on these four outcome variables. Regarding the sustainable collections' design and production, managerial implications are described as the need to communicate the ethical approach and efforts.