There is evidence that lower cognitive functioning goes along with lower self-regulated health behaviour. Planning has been shown to be an effective strategy to successfully implement one's behavioural intentions into behaviour. Thereby, planning is assumed to promote automatic behavioural responses and thus unfolding its beneficial effects without drawing on cognitive resources. Thus, the aim of this study was to examine whether participation in a planning intervention would compensate for the potential negative effects of lower cognitive functioning on change in dietary behaviour. The sample consisted of 144 overweight and obese participants. Half of the sample participated in a planning intervention. Fat consumption and cognitive functioning was assessed at baseline. Six months later, fat consumption was assessed again. Results indicate that despite an unexpected non-significant effect of the planning intervention on change in fat consumption, a significant interaction between planning intervention and cognitive functioning emerged: Cognitive functioning was unrelated to change in fat consumption in the planning group, but in the control group individuals with lower cognitive functioning were more unsuccessful to reduce their fat consumption. This study provides first evidence that planning compensates lower cognitive functioning in an experimental field study in the context of dietary change.