The Buy Now Pay Later (BNPL) mechanism has emerged as a major force in the digital payments landscape, transforming consumers’ online transactions worldwide. With the rise of online shopping and the emergence of e-commerce, consumer purchase behaviour has shifted from rational to impulsive purchases. In addition, more and more online retailers and platforms are partnering with payment solution providers to offer their customers a simple and seamless payment experience. However, there are concerns about the potential negative effects of the user-friendly BNPL system, especially in terms of impulsive purchasing behaviour. This study focuses on Malaysian Millennials’ impulsive purchase behaviour using the BNPL system. It proposed a research model based on the Stimulus Organism Response (S-O-R) framework to investigate consumer response and behaviour.