Purpose This study aims to expand the theory of planned behaviour (TPB) to understand determinants of financial inclusion participation behaviour through the mediating effect of Islamic finance product (IFP) adoption. Design/methodology/approach A quantitative research design was deployed using primary data from a survey conducted within the Muslim-dominated regions in Nigeria, which was analysed using partial least squares structural equation modelling. Findings It was found that the original TPB variables, attitude, subjective norms, perceived behavioural control (PBC) and behavioural intention have strong positive influences on financial inclusion participation behaviour, however, among the new variables, government support and IFPs adoption directly influence, while awareness and access to banking and digital channels were not. Furthermore, IFPs adoption significantly mediates the relationship between attitude, behavioural intention, government support and access to banking and digital channels and financial inclusion participation, but it failed to mediate that of subjective norms, PBC and awareness. Research limitations/implications These findings imply the need to establish more Islamic financial institutions or conventional banks to introduce IFPs in Muslim-dominated regions in Nigeria, as such products are desirable in expanding financial inclusion. While such is being pursued, policymaking bodies responsible for financial inclusion should design appropriate programmes to create awareness of IFPs for expanding financial inclusion. Originality/value To the best of the authors’ knowledge, this study could be the first to expand the TPB by integrating IFP adoption as a mediator within the context of financial inclusion participation as well as the incorporation of awareness, government support and access to banking and digital channels as additional variables.