Previous research has identified the presence of an attitude-behaviour gap in sustainable consumerism. To date, the research assessing this attitude-behaviour gap in fashion sustainable consumption is still sparse. Using mixed methods, the present study aims to further investigate the attitude-behaviour gap in sustainable fashion consumerism in Millennials. Study 1 used qualitative data analysis (focus groups) to identify barriers to sustainable fashion consumerism. Based on these outcomes, the Perceived Knowledge and Availability of Sustainable Fashion Scale has been developed and validated in Study 2. Then, Study 3 was used to quantify several variables such as Knowledge and Accessibility (using the scale developed in Study 2) as well as Equity, Authenticity, Functionality, Localism, Exclusivity, Sufficient Frugal Consumption, Purchase Choice, and Locus of Control. Our results suggest that knowledge about sustainable fashion is a crucial factor in the attitude-behaviour gap in sustainable fashion consumption. Accessibility was also identified as a factor, but this relationship is more complex. It is suggested that knowledge is the main factor in the attitude-behaviour gap and that once knowledge is controlled for, accessibility will become the main factor. Theoretical and practical implications are discussed in the present manuscript.