The rapid growth of e-commerce has intensified packaging waste, highlighting the need for sustainable alternatives. Reusable transport packaging (RTP), rooted in circular economy principles, presents a promising solution but faces challenges in adoption within the German online retail market. Addressing a gap in theory-driven research on RTP, this study extends the theory of planned behavior (TPB) by integrating environmental concern, personal innovativeness, perceived usefulness, return convenience, and shopping frequency to explain consumers’ intention to adopt RTP. Survey data from 792 German online shoppers were analyzed using partial least squares structural equation modeling. The results show that environmental concern and perceived usefulness are key drivers of adoption intention, whereas personal innovativeness and return convenience have no effect. Mediation analyses reveal that attitude and perceived usefulness fully mediate the impact of return convenience on intention and partially mediate the effect of environmental concern. Shopping frequency does not impact the relationship between attitude or perceived usefulness and intention, but it moderates the environmental concern-intention relationship, weakening its influence among frequent shoppers. Theoretically, the study advances TPB by incorporating underexplored psychological and situational factors relevant to low-complexity sustainable innovations. Managerially, it suggests that retailers and packaging providers should emphasize the functional and environmental benefits of RTP to target both environmentally conscious and habitual online shoppers.