Only a few industries face challenges regarding sustainability to the same extent as the fashion industry being one of the largest consumer industries and the second most polluting industry in the world. These challenges are encouraged by the fast fashion business model, which is dominant in the sector and relies on the quick responsiveness to latest fashion trends while maintaining low prices, encouraging more consumption and shorter product life cycles. On the other side, a consumer shift towards more environmental consciousness can be observed leading to most fast fashion brands integrating sustainability in various forms into their products and services. A path that many fast fashion brands take in order to become more sustainable is the strategy of green branding through green brand extensions. Green brand extensions (GBEs) involve the application of an established brand name to new and greener products due to environmental considerations of the brand and can be divided into green line extensions (GLEs) and green category extensions (GCEs). Due to identified research gaps concerning how green brand extensions change consumer attitudes as well as the relationship between brand loyalty and the green image of brands, the following research question got developed to gain in-depth in consumer behavior in the fast fashion industry related to green brand extensions: RQ: How do green brand extensions of fast fashion brands affect the consumers’ parent brand attitude and brand loyalty? This primary study that aims to answer the research question was conducted following a deductive research approach and adopting a quantitative research design through collecting data by means of a web questionnaire. Thereby, the brand attitude of fashion consumers on green line and green category extensions as well as the relationship to brand loyalty got investigated focusing on Generation Y and Z as largest consumer groups of fast fashion. The main results of the study are that the introduction of a GLE by a fast fashion brand generally leads to a more positive brand attitude and a slightly improved brand loyalty of consumers towards the parent brand. However, when introducing a GCE, the launch has rarely a positive effect on the brand attitude and brand loyalty of consumers towards the parent brand. The relationship of GLEs and brand attitude is thereby moderated by the environmental concern of consumers as well as their friends and by the initial brand attitude towards the fast fashion brand. The relationship of GLEs and brand loyalty on the other hand is only moderated by environmental concern of consumers and the initial brand attitude. Looking at GCEs and their relationship with brand attitude as well as brand loyalty, it is influenced and moderated by the age of consumers, the environmental concern of their friends as well as consumers’ initial brand loyalty towards the parent brand.