OBJECTIVE: Price promotions influence food choices, and are disproportionately applied to foods high in fat, sugar, and salt (HFSS), consumption of which is a known contributor to poor diet and diet-related non-communicable diseases. This study evaluates the effectiveness of (1) restricting price promotions of HFSS products and (2) allowing such promotions but restricting communications regarding them, in changing purchasing behaviours. DESIGN: Between-subjects randomised controlled trial with three arms: (1) reduced price, labelled a price promotion ("Usual Practice"); (2) reduced price, not labelled a price promotion ("No Communication"); (3) standard price ("No Promotion"). Participants selected food and drinks for 2 days in a one-off shop, without restrictions on the number of items to purchase or the budget, at a simulated online supermarket. We measured total energy (kcal) in basket, total cost, total number of items, energy density (kcal/100g), and proportion of the basket that was HFSS. SETTING: Simulated online supermarket. PARTICIPANTS: Adults representative of the UK population (n=9,004). RESULTS: There were no significant differences in energy in baskets between groups, after controlling for age, gender, household income, ethnicity, and deprivation. No significant differences were found between groups' baskets for total cost, energy density, or proportion HFSS. Participants in the "Usual Practice" group selected significantly more items than the "No Promotion" group (11.4 vs. 11.1; p=.003). CONCLUSIONS: No significant effects of restricting price promotions, or communications of them, were found for purchasing behaviours in a simulated online supermarket. Further price-based interventions, especially in real-world settings, are needed to provide robust policy recommendations.