Luxury fashion firms increasingly incorporate the principles of environmental sustainability into their supply chain strategies in response to a growing demand for sustainable products. Starting from this premise, the present study explores the relationship between upstream vertical integration and environmental sustainability in the luxury fashion industry. Based on data from a survey and a set of interviews, this paper examines the link between firm size and the importance of environmental sustainability as a driver of upstream integration. In addition, it investigates the connection between the degree of upstream integration and manufacturers’ environmental sustainability performance. According to the research results, large corporations attach greater importance than SMEs to environmental sustainability when considering an upstream integration process. Also, higher levels of vertical integration are positively associated with better sustainability performances.