In this manuscript, we explore the implementation of sustainable marketing orientation (SMO) within the startup context. SMO is modelled as a three-dimensional formative construct comprising strategic integration, societal engagement, and ethical capabilities. In addition, in this study, we expand knowledge by extending the SMO literature and building ties between the theory of planned behaviour and sustainable marketing theoretical perspectives, by integrating attitudes, subjective norms, behavioural control of managers, and implementation of SMO in a single model. With the analysis of structural equation modelling, we confirmed the three-dimensional SMO and its contribution to start-ups' profitability. Moreover, we proved that attitudes are marginal, whereas subjective norm and perceived behavioural control are a very strong predictor of SMO. Finally, by using the cluster analysis, we revealed that B2B start-ups have lower levels of strategic integration, societal engagement, and ethical capabilities, as opposed to B2C start-ups.