Globalisation has led to a rising need for sustainable efforts, not least in the fast fashion industry which is associated with unsustainable procedures in order to meet consumers’ needs and desires. In the fast fashion industry, collections of ecological fabrics, fair working conditions as well as higher quality are examples of sustainable actions. Additionally, the increasingly globalised world has created a demand to comprehend how the deeply rooted national culture affects consumers in the international fast fashion industry. The main target segment of the fast fashion industry is the Millennial generation and should thereby be further emphasised. The relevance of these topics led to the formulation of following research question: How does national culture affect Millennials’ perception of sustainability and intended buying behaviour in the fast fashion industry? The literature review introduces the five identified themes which are considered relevant in relation to the area of research; the Millennials, sustainability, fast fashion, sustainable consumer behaviour and culture. To finalise the literature review, a conceptual framework is presented which explains the connection between the themes. Furthermore, the thesis adopts a qualitative research method with an abductive approach. Therefore, four cross-cultural focus groups of 28 Millennials with seven different nationalities are conducted. This method is considered the most suitable for this thesis’ research area and purpose. Further, the empirical findings and the analysis are combined into one chapter which creates a discussion between the literature review, especially Hofstede’s dimensions of national culture, and the gathered data. Finally, the conclusion includes the answer to the research question, theoretical and practical implications, limitations and suggestions for further research. The results of this thesis indicate that national culture affects the perception of sustainability and the intended buying behaviour in the fast fashion industry in a complex manner, since the impact of national culture depends on the combination of dimensions. Furthermore, certain dimensions appear to be more dominant and decrease the impact of others.