Free-roaming cats negatively affect wildlife, human health, and society, and anthropogenic food sources partly maintain their populations. There is a dearth of theory-informed interventions to change people’s beliefs about feeding animals. Here, we outline a behavioural change intervention protocol to modify Malaysians’ key beliefs (i.e. the most influential beliefs) about feeding free-roaming cats. Our protocol serves as a novel, timely, and potentially valuable tool for addressing a significant conservation and societal issue. The Theory of Planned Behaviour is the theoretical framework of the intervention, underpinning its targets (i.e. behavioural beliefs, normative beliefs), content, delivery, and evaluation. The prescriptive intervention consists of one full-day workshop (duration = 5 h) with three sessions each attempting to alter one key belief using behavioural change strategies. A two-armed parallel-group prospective-cluster randomised controlled trial will be used to evaluate the efficacy of the intervention. The protocol can be easily delivered for the public and adapted for other types of locations, human–animal interactions, and contexts. It also complements animal management and policy change approaches.