BackgroundPictorial health warnings on sugar-sweetened beverages (SSBs) are a promising policy for preventing diet-related disease in children. A recent study found that pictorial warnings reduced parents’ purchases of SSBs for their children by 17%. However, the psychological mechanisms through which warnings affect parental behavior remain unknown. We aimed to identify the mechanisms that explain how pictorial warnings affect parents’ SSB purchasing behavior for their children using secondary data from a randomized trial.MethodsIn 2020–2021, parents of children ages 2 to 12 years (n= 325) completed a shopping task in a convenience store laboratory in North Carolina, USA. Participants were randomly assigned to a pictorial warnings arm (SSBs displayed pictorial health warnings about type 2 diabetes and heart damage) or a control arm (SSBs displayed a barcode label). Parents then bought a beverage for their child and took a survey measuring 11 potential psychological mediators, selected based on health behavior theories and a model explaining the impact of tobacco warnings. We conducted simple mediation analyses to identify which of the 11 mechanisms mediated the impact of exposure to pictorial warnings on purchasing any SSBs for their children.ResultsTwo of the 11 constructs were statistically significant mediators. First, the impact of pictorial warnings on the likelihood of purchasing any SSB was mediated by parents’ perceptions that SSBs were healthier for their child (mediated effect= −0.17; 95% CI = − 0.33, − 0.05). Second, parents’ intentions to serve SSBs to their children also mediated the effect of warnings on likelihood of purchasing any SSB (mediated effect= −0.07, 95% CI=-0.21, − 0.003).ConclusionsPictorial warnings reduced parents’ purchases of SSBs for their children by making parents think SSBs are less healthful for their children and reducing their intentions to serve SSBs to their children. Communication approaches that target healthfulness perceptions and intentions to serve SSBs may motivate parents to buy fewer SSBs for their children.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12966-023-01469-3.