Gambling messaging is one potential component of a public health approach towards reducing gambling-related harm (GRH). This systematic review examined the effectiveness of existing public health gambling messaging interventions. Four databases were searched from inception to June 2023. Twenty-one papers met inclusion criteria. Studies were conducted in diverse settings, employed various designs, and evaluated messaging interventions that differed in their aims and reported outcomes. Interventions were categorised into three groups: general population-level messaging, gambling harm reduction messages, and product risk warnings. The evidence—currently limited—suggests that existing gambling messaging interventions are ineffective in changing behaviours but can raise awareness of GRH. Our findings underpinned recommendations for how the format and presentation of currently used ‘safer’ gambling messages and product warning labels may be improved. Research is required to develop and test messages that can help prevent and reduce GRH both for the general population and people who gamble.