Sustainable consumer behaviour has become a priority worldwide as unsustainable consumption patterns continue to accelerate environmental degradation. While traditional economic models often assume rational decision-making, evidence shows us that consumers tend to rely on cognitive shortcuts, their emotions, as well as social influences. These factors help shape their everyday choices in ways that are not purely reasoned. Scholars in behavioural economics highlight how defaults (Thaler and Sunstein), emotions and social norms (Cialdini, Opower studies), and simplified design approaches (eco-labelling, minimalist packaging) can significantly influence behaviour without restricting the freedom of choice, which consumers usually consider a strong determining factor of consumption. However, these insights are rarely connected systematically to product design, which is where the problem arises. Building on this foundation, this paper explores how behavioural economic principles can be applied to encourage sustainable consumer behaviour through design. This research paper is limited to secondary data collection, peer-reviewed literature, and behavioural theories rather than primary data, which could include biases in such a sensitive topic. It also includes case studies such as Alibaba’s “no cutlery” default, energy-saving appliance settings, and the EU energy label. These real-world examples help me demonstrate how defaults, emotional and social reinforcement, and simplicity can guide consumers toward sustainability by embedding these behaviours into everyday interaction with products. Therefore, I argue that by strategically integrating behavioural economic principles into product design, firms can make sustainable behaviour the most effortless, emotionally rewarding, and socially reinforced choice, thereby generating lasting environmental and economic value.