This paper explores how online social interactions may influence consumers’ impulse purchase when they browse online group shopping websites. We develop a research model based on the stimulus-organism-response framework and prior research on online social interactions. We propose that social interaction factors may serve as stimulating cues. In specific, opinion-based social interaction (i.e., review quality and source credibility) and behavior-based social interaction (i.e., observational learning) may affect perceived usefulness and positive affect (organism), which further lead to consumers’ urge to buy impulsively (response). We empirically test our model using an online survey method. The results show that both source credibility and observational learning positively influence perceived usefulness and positive affect. Meanwhile, review quality has a significant impact on perceived usefulness, but not on positive affect. Perceived usefulness is found to posit a significant impact on positive affect, which influences urge to buy impulsively. We believe that these findings have important implications for both research and practice. We provide new insights to understand online impulse buying by incorporating the perspective of online social interactions. We also expect that new e-business strategies can be developed for online retailers regarding how to evoke consumers’ impulse purchase urges.