The main objective of this study was to assess whether self-efficacy (SE) could function as a moderator of the effect of a tailored Internet-based intervention aimed at increasing self-reported diabetes self-care behaviours. In a two-group, 1-month interval pre-test-post-test randomized controlled trial, participants (N = 64) were assigned at random to either a group that received an intervention on the area of self-care (blood glucose monitoring, diet management or physical activity) for which the reported SE was lowest (LSE group) or to a group that received an intervention on area of self-care for which the reported SE was highest (HSE group). Improvements in self-care were observed for both groups, but the HSE group improved more. Self-care also increased for those areas that the intervention did not target. Furthermore, SE levels decreased from baseline to follow-up. This study suggests that SE can function as a moderator in a behavioural intervention for diabetes self-care, and hence that initial level of SE provides relevant information for tailoring such interventions.