© Objectives Conduct a head-to-head experimental test of responses to alcohol harm reduction advertisements developed by alcohol industry Social Aspects/Public Relations Organisations (SAPROs) versus those developed by public health (PH) agencies. We hypothesised that, on average, SAPRO advertisements would be less effective at generating motivation (H1) and intentions to reduce alcohol consumption (H2) but more effective at generating positive perceptions of people who drink (H3). Design Online experiment with random assignment to condition. Participants 2923 Australian adult weekly drinkers (49% high-risk drinkers) recruited from an opt-in online panel. Interventions Participants viewed 3 of 83 advertisements developed by PH agencies (n=2174) or 3 of 28 advertisements developed by SAPROs (n=749). Primary outcome measures Participants reported their motivation to reduce the amount of alcohol consumed; behave responsibly and/or not get drunk; and limit their drinking around/never supply to minors, as well as intentions to avoid drinking alcohol completely; reduce the number of drinking occasions; and reduce the amount of alcohol consumed per occasion. Participants also reported their perceptions of people who drink alcohol on six success-related items and four fun-related items. Results Compared with drinkers exposed to PH advertisements, those exposed to SAPRO advertisements reported lower motivation to reduce the amount of alcohol consumed (β=-0.091, 95% CI-0.171 to-0.010), and lower odds of intending to avoid alcohol completely (OR=0.77, 0.63 to 0.94) and to reduce the amount of alcohol consumed per occasion (OR=0.82, 0.69 to 0.97). SAPRO advertisements generated more favourable fun-related perceptions of drinkers (β=0.095, 0.013 to 0.177). Conclusions The alcohol harm reduction advertisements produced by alcohol industry SAPROs that were tested in this study were not as effective at generating motivation and intentions to reduce alcohol consumption as those developed by PH organisations. These findings raise questions as to whether SAPROs should play a role in alcohol harm reduction efforts.