Sustainable fashion is becoming increasingly integrated in today's society. However, the use of the term “eco” in fashion branding may evoke negative consumer perceptions, especially in terms of the perceived fashionability of clothing items. Hence, this study investigates consumer responses to environmental versus social sustainability cues used by an online fashion shop. In particular, the moderating influence of basic versus modern fashion style is examined, considering the idea that fashion style is central to consumers' brand evaluations. Using a 2 (environmental vs. social sustainability cue) × 2 (timeless basic vs. modern fashion style) between‐subjects factorial design, this study explored the effectiveness of sustainability cues on fashion brand value perceptions (emotional, social, and functional value) and brand attitudes. Among the 358 respondents, the results indicate more positive brand attitudes when fashion items are presented with an environmental (vs. social) sustainability cue. In particular, this effect is stronger for the timeless basic (vs. modern trend‐based) fashion style. Yet, a cue referring to environmental sustainability increases social brand values for the modern fashion style. This study offers valuable contributions to the sustainable fashion literature and recommendations for online fashion retailers.