Fast fashion retailers bring apparel products to market much more quickly than in traditional apparel retailing. Fast fashion retailers are able to respond quickly to both fashion trends and consumer demand. Yet the emphasis on speed has quickened the consumption of fast fashion apparel products, which are produced with low quality and thus have short product lifespans. Critics of fast fashion cite these negatives, combined with chemicals, water and energy used in production, among other issues, as detrimental to the environment. However, some fast fashion retailers have implemented sustainable options into their product offerings. This study analyzes consumers’ perceptions of a sustainable brand extension introduced by a fast fashion retailer. The research is divided into two studies. First, an exploratory study was conducted to assess consumer knowledge of sustainability and fast fashion and to uncover potential factors for the model of the second study. Findings show a low level of knowledge of the holistic principle of sustainability and specific adverse effects of the apparel industry and of the concept of fast fashion. Despite some skepticism, participants feel steps must be taken towards sustainability and that every effort helps. The second part of the research tested consumer perceptions of a potential sustainable line extension introduced by a specific fast fashion retailer. Following brand extension theory, study two tests the influence of brand knowledge and affect on both the perceptions of brand-cause fit and brand-extension fit and the influence of cause knowledge and involvement on the perceptions of fit between brand and cause. The influence of brand-extension fit and brand-cause fit on attitude toward the extension was also analyzed. An online self-administrated survey using the written scenario approach resulted in 598 responses. Findings show the influence of brand knowledge and affect on brand-extension fit and brand-cause fit and cause knowledge and involvement on cause-brand fit. Implications for retailers include leveraging consumers’ past knowledge and affect of the brand through marketing of the sustainable product. Overall the study shows that consumers do view sustainable products as fitting with fast fashion retailers, based on their previous knowledge and affect of the brand and cause.