Background: In response to public concerns and campaigns, some UK supermarkets have implemented policies to reduce less healthy food at checkouts. We explored the effects of these on purchases of less healthy foods commonly displayed at checkouts. Methods and findings: We used a natural experimental design and two data sources providing complementary and unique information. We analysed data on purchases of small packages of common, less healthy, checkout foods (sugary confectionary, chocolate and potato crisps) from 2013-17 from nine UK supermarkets (Aldi, Asda, Co-op, Lidl, M&S, Morrisons, Sainsbury’s, Tesco and Waitrose). Six supermarkets implemented a checkout food policy in 2013-17 and were considered intervention stores; the remainder were comparators. Firstly, we studied the longitudinal association between implementation of checkout policies and purchases taken home. We used data from a large (n~30,000) household purchase panel of food brought home to conduct controlled interrupted time-series analyses of purchases of less healthy common checkout foods from 12 months before to 12 months after implementation. We conducted separate analyses for each intervention supermarket, using others as comparators. We synthesised results across supermarkets using meta-analyses. Implementation of a checkout food policy was associated with an immediate reduction in 4-weekly purchases of common checkout foods of 157,000 (72,700-242,800) packages per percentage market share – equivalent to a 17.3% reduction. This decrease was sustained at one year with 185,100 (121,700-248,500) fewer packages purchased per 4-weeks per percentage market share – equivalent to a 15.5% reduction. The immediate, but not sustained effect, was robust to sensitivity analysis. Secondly, we studied the cross-sectional association between checkout food policies and purchases eaten without being taken home. We used data from a smaller (n~7,500) individual purchase panel of food bought and eaten ‘on the go’. We conducted cross-sectional analyses comparing purchases of common checkout foods in 2016-17 from supermarkets with and without checkout food policies. There were 76.4% (95% CI: 48.6 to 89.1%) fewer annual purchases of less healthy common checkout foods from supermarkets with, versus without, checkout food policies. The main limitations of the study are that we do not know where in store purchases were selected from and cannot determine the effect of changes in purchases on consumption. Other interventions may also have been responsible for the results seen. Conclusions: There is a potential impact of checkout food polices on purchases. Voluntary supermarket-led activities may have public health benefits.