Abstract: Fashion industry to be one of the largest consumer industries & the second most polluting in the world because only a few industries confront as many sustainability issues as the fashion industry. This sustainability challenge is increasing day by day due to the nature of the fast fashion model, which have accelerated the consumption of fast fashion clothing goods made of poor quality and have short product lifespans. On the other hand, due to consumers’ and society’s rising demand for sustainable fashion products, fast fashion companies are investing heavily in integrating sustainability into their products or services. Most of these companies are adopting sustainable brand extension strategies for sustainable branding. The purpose of the study is to assess how younger Indian consumers’ parent brand attitudes are affected in the case of sustainable brand extension introduced by fast fashion brands. To address this purpose, a literature review was conducted before conducting the primary study by using a quantitative research design via a self- administered questionnaire. The result of primary research indicated that focusing on sustainable branding with a brand extension strategy is not misguided since it positively impacts brand attitude. The result of primary research indicated that focusing on sustainable branding with a brand extension strategy is not misguided since it positively impacts brand attitude. In addition, based on the multiple regression analysis, it concluded that all of the selected variables {brand influence (brand knowledge & brand affect), attitude towards sustainable brand extension (ATSBE), and consumer influence (fashion interest, social belonging, and sustainability concern)} are significantly correlated with brand attitude. In this process, brand influence factors are considered the most important in terms of affecting brand attitude. Therefore, for the effectiveness of sustainable brand extensions, companies should focus on not only brand extension fit but also brand factors such as brand knowledge and brand affect.