<p>Transitioning to more plant-based diets is critical for both health and environmental sustainability, yet consumer adoption remains limited. Supermarkets, as key food providers, are well positioned to facilitate this shift. Positioning of retail assortments is a powerful way to create a choice architecture that facilitates consumers’ sustainable product choices. This study tested a complement-based product placement strategy designed to promote plant-based protein purchases by aligning products with meal preparation goals, rather than dietary preference or product type. Over a nine-week period, ten Belgian supermarkets positioned four plant-based protein products alongside six ready-to-cook meal kits, supported by recipe leaflet to inspire meal preparation. To evaluate the impact, we analysed sales data using a difference-in-differences model based on a negative binomial distribution, comparing results to a 5-week pre- and 4-week post-intervention period and ten control stores within the same supermarket chain. The statistical model did not yield statistically significant effects of the intervention. A process evaluation revealed variability in implementation across stores due to differences in infrastructure and competing priorities, which may have influenced the intervention’s effectiveness. Further barriers were product-related challenges and misalignment with customer preferences, while facilitators included staff collaboration and interest in more effective promotion strategies. This study adds to the growing evidence on behaviourally informed retail strategies. Despite mixed outcomes, the findings highlight both the potential and challenges of these interventions in supporting the protein transition. Further research into effective and practical behaviourally informed supermarket interventions is needed to promote sustainable food choices and support broader dietary change.</p>