Corruption in the form of bribery continues to be a major societal challenge around the world. The current lab-in-the-field study tested whether dynamic descriptive norms messages on posters can help to reduce bribery. Before, during and after placing posters throughout a medium-sized South African town, incentivized measures of social norms and bribery were assessed in a mobile lab. A total of 311 participants stemming from the general population took part. In line with the pre-registered predictions, the results reveal that people: (1) perceive bribery to be less common; and (b) engage in bribery in a corruption game less frequently when the posters were displayed. The discussion outlines how social norms nudging campaigns can be leveraged to spur collective action against corruption.