Seamlessness experience has become one of the most vital experiences throughout the customer journey across multiple touchpoints and shopping stages in an omnichannel context. But the effects of the seamlessness experience on important customer engagement behavior, including customer purchase behavior, customer knowledge behavior, customer referral behavior, and customer influencer behavior, remain unexplored. The purpose of this study is to demystify the link between the journey seamlessness experience and customer engagement behavior, together with the mediating role of self-brand connection, as well as the moderating role of the feeling of groundedness between journey seamlessness experience and self-brand connection. Based on 474 usable questionnaires collected from omnichannel customers in Ho Chi Minh, the partial Least Squares Structural Equation Modeling (PLS-SEM) approach is applied to analyze and test the proposed hypotheses. The analysis result reveals that seamlessness experience has the strongest effect on customer purchase behavior, followed by customer knowledge behavior, customer influencer behavior, and then customer referral behavior. In addition, the mediating role of self-brand connection on the nexus between seamlessness experience and customer engagement behavior is supported-which means the transformation from omnichannel experience into engagement behavior requires the development of a strong self-brand connection. Furthermore, the effect of journey seamlessness experience on self-brand connection is intensified among consumers who have strong feelings of groundedness. These results provide important implications for omnichannel retailers who would like to improve their customers’ self-brand connection and subsequent engagement behavior