The LiveLighter® 2016 'Junk Food' campaign ran for 5 weeks in Western Australia (WA) and urged adults to reduce their junk food consumption to avoid weight gain and associated chronic disease. A cohort design assessed campaign impact on knowledge, beliefs and behaviour. Pre-campaign (n = 1501) and post-campaign (n = 737) telephone surveys of WA adults aged 25-49 were undertaken. A post-campaign-only sample (n = 501) controlled for pre-test effects. The campaign reached 57% of respondents and increased knowledge of the link between overweight and non-alcoholic fatty liver disease (78% cf. 87%; P < 0.01). Following campaign activity, respondents were more likely to avoid purchasing food from settings depicted in the advertisements (fast food outlets: 63% cf. 77%; service stations: 82% cf. 93%; vending machines: 87% cf. 96%; all P < 0.001), particularly among pre-campaign consumers of fast food and salty snacks ([less than or equal to]2 times/week). Weight-loss self-efficacy increased among adults with higher body weight and campaign exposure (89% cf. 97%; P < 0.01), and there was no increased endorsement of overweight stereotypes among the full sample. Support for fiscal disincentives and nutritional disclosure policies increased among healthy weight adults. LiveLighter® positively influenced knowledge, beliefs and behaviours, along with attitudes towards policies to encourage healthy eating, supporting LiveLighter® as an advocacy campaign.