Fast fashion production and consumption have undeniably contributed to environmental challenges, including climate change. At the same time, research on sustainable consumption consistently shows a gap between consumers’ pro-environmental intentions and their actual purchasing behavior. Drawing on consumer behavior theory and innovation resistance theory (IRT), this study examines the psychological barriers and functional barriers that shape the intention–behavior gap. We integrate environmental concern and sustainability knowledge with IRT to explore the impact of these drivers and barriers on Gen Z consumers. We analyse data from 684 respondents in the United States, India, and Malaysia using PLS-SEM and fsQCA. The results show that environmental concern and sustainability knowledge positively influence the intention to adopt sustainable fashion clothing (SFC), while image barriers reduce the likelihood of adoption. Although examined within the apparel sector, the study highlights mechanisms of resistance and consumer psychology that are relevant to broader sustainable consumption contexts.