The growing global focus on sustainability has redefined consumer decision-making, compelling businesses to align marketing strategies with environmental values. This study examines the effect of sustainability perception and green marketing on buying behaviour among Millennials and Gen Z consumers in India, with a specific focus on the mediating role of attitude toward sustainability. Using quantitative methods and SPSS v27 for analysis, data from 965 respondents were tested through reliability, validity, regression, and mediation models (PROCESS Macro, Model 4). Results revealed that sustainability perception significantly influences buying behaviour, while green marketing efforts enhance this relationship. Attitude toward sustainability was found to partially mediate the perception–behaviour link. A one-way ANOVA further identified generational differences, with Gen Z exhibiting higher sustainability consciousness and purchase intent than Millennials. The findings extend the Theory of Planned Behaviour to the sustainability domain and offer managerial insights into the design of effective, authentic, and generation-specific green marketing strategies. The study contributes to the understanding of how cognitive, affective, and contextual factors interact to shape sustainable consumption in emerging markets