Behavioural change approaches are often used to address societal challenges. However, the resulting behavioural interventions often evoke inconsistent and heterogeneous results. Ambiguity is a potentially key lens in helping to explain different interpretations of singular behavioural interventions, and hence unexpected outcomes, yet remains overlooked in current behavioural change research. In response to this, we take a case study approach to examine the role of ambiguity in designing behavioural interventions in a highly ambiguous context. We conduct a thematic analysis of the data collected, and identify three major types of ambiguity that each evoke heterogenous responses in specific ways. We synthesise the results in the form of the ‘Ambiguity in Behavioural Design framework’. Our framework highlights the need to acknowledge ambiguity when designing for behavioural change and is the first to identify this element within the field of behavioural design. It sets a foundation for understanding ambiguity in behavioural design and has implications for design theory and practice.