As climate change continues, environmental sustainability has become a popular topic among brands and consumer groups. The fashion industry has detrimental impacts on the natural environment; however, little is known about how brand benefits can help sustainable fashion brands develop relationships with consumers and promote consumer behavior. This study focuses on Instagram to investigate how consumers’ perceived brand benefits predict relationship commitment, electronic word-of-mouth (eWOM), and purchase intention. Prior studies have overlooked the possible effects of various benefits. This study outlines five benefits of sustainable fashion brands: inner self-expression, social self-expression, warm glow, green, and economic benefits. Results from a survey of sustainable fashion brand followers on Instagram showed that eWOM positively related with economic benefits and negatively with warm glow and green benefits. Findings further indicated a mediating effect of relationship commitment between benefits and consumers’ behavior. Lastly, the level of environmental attitude influenced the mediating impact of relationship commitment. The implications of these findings are discussed, and suggestions for future research are provided.