Indonesia is experiencing a demographic bonus, where millennials make up a significant portion of the productive age population. This demographic shift presents challenges in meeting labour market demands, as well as consumption patterns, particularly concerning sustainable consumption. Simultaneously, environmental issues such as climate change and unsustainable resource usage have become pressing global concerns. Millennials have a crucial role in addressing these challenges by adopting environmentally friendly behaviours, including green product consumption. However, the factors that drive millennials' green purchase intentions, particularly in Indonesia, remain understudied. This study aims to investigate the influence of environmental knowledge, environmental concern, social influence, and attitude on the green purchase intentions of Indonesian millennials. The research employs a quantitative descriptive approach, using purposive sampling to collect data from 150 millennials in Banjarmasin. Data were analyzed using Structural Equation Modeling (SEM) through the Partial Least Squares (PLS) method. The findings reveal that environmental concern indirectly affects green purchase intention through the mediating role of attitude. Additionally, social influence and environmental knowledge have significant positive impacts on both attitude and green purchase intention. The study highlights the importance of enhancing environmental knowledge and leveraging social influence to encourage sustainable purchasing behaviour among millennials. Marketers can use these insights to design effective strategies that promote eco-friendly products to this demographic. Future research could explore other factors, such as perceived product quality and price sensitivity, to gain a more comprehensive understanding of green consumer behaviour.