Retailers and consumers are increasingly “omnichannel”. This means that retailers offer multiple integrated offline and online channels to their customers, while consumers use multiple offline and online channels throughout their shopping journeys. In these shopping journeys, consumers can travel for researching, testing, receiving and returning activities related to a purchase, next to the purchasing itself. It is unclear how such omnichannel consumer behaviour materialises in practice. This information is important for practitioners from retail as well as for society, not in the least because of the environmental impact that shopping trips generate. Existing environmental assessments of retail-related transport and logistics do not account for consumers’ omnichannel shopping and travel behaviour. To fill this gap in research, we set up a case-study collaboration with an omnichannel footwear retailer in Belgium. We collected data on logistics and consumer flows and analysed this data to determine the CO2 footprint. Our research results in six profiles, of which “the online shopper” that shops online and receives its purchase at home or at a collection point generates the lowest impact. However, when online shoppers travel to stores prior to their e-purchase and become “showroomers”, the external CO2 costs double compared to “traditional shoppers” that carry out all shopping activities in-store and are more than eight times higher compared to “online shoppers”. Although the case-study context should be taken into account (e.g., in terms of product type, retailer type and geography), a sensitivity analysis demonstrates the robustness of our results.