The public health impact of health behaviour interventions is highly dependent on large-scale implementation. Intermediaries-intervention providers-determine to a large extent whether an intervention reaches the target population, and hence its impact on public health. A cross-sectional study was performed to identify the correlates of intermediaries' intention to implement a computer-tailored physical activity intervention. According to theory, potential correlates are intervention characteristics, organisational characteristics, socio-political characteristics and intermediary characteristics. This study investigated whether intermediary characteristics mediated the association between the intervention, organisational and socio-political characteristics and intention to implement the intervention. Results showed that intervention characteristics (i.e., observability (B = 0.53; p = 0.006); relative advantage (B = 0.79; p = 0.020); complexity (B = 0.80; p < 0.001); compatibility (B = 0.70; p < 0.001)), organisational characteristics (i.e., type of organization (B = 0.38; p = 0.002); perceived task responsibility (B = 0.66; p ≤ 0.001); capacity (B = 0.83; p < 0.001)), and the social support received by intermediary organisations (B = 0.81; p < 0.001) were associated with intention to implement the intervention. These factors should thus be targeted by an implementation strategy. Since self-efficacy and social norms perceived by the intermediary organisations partially mediated the effects of other variables on intention to implement the intervention (varying between 29% and 84%), these factors should be targeted to optimise the effectiveness of the implementation strategy.