The consumption of tobacco, alcohol and ultra-processed foods and beverages drives the global noncommunicable disease (NCD) crisis in Pacific small island developing states (PSIDS). Addressing the commercial determinants of health (CDoH) requires policy coherence across government sectors; however, entrenched neoliberal ideologies that exhort self-regulation of markets and market actors impede such efforts. This paper aims to explore the roles ideas play in governing CDoH, through the analysis of causal ideas in multisectoral tobacco governance in Fiji and Vanuatu. An explorative, qualitative case study design was applied. Data collection relied primarily on in-depth interviews, of which 70 were conducted between 2018 and 2019. Data analysis was guided by a theory-informed analytical framework. Two causal ideas influence multisectoral tobacco governance in Fiji and Vanuatu. According to the idea of individual responsibility, high smoking prevalence is the consequence of individualsâ€™ unhealthy lifestyle choices; it nominates the Ministry of Health as the responsible actor to solve this issue by providing health education. In contrast, the idea of CDoH argues that harmful commodity industries drive the NCD epidemic, and the sectors that regulate these private actors should be kept in closer check to ensure that their policies are aligned with the objectives of public health. In Fiji and Vanuatu, the non-health government agencies are effectively excused from implementing multisectoral tobacco policies because the dominant idea of individual responsibility relieves them of any responsibility. The wider adoption of the idea of CDoH is needed in PSIDS to tackle the NCD crisis.