Background: Most theories of health-behavior change focus exclusively on individual self-regulation without taking social factors, such as social support, into account. This study's first aim was to systematically test the added value of received instrumental and emotional social support within the Health Action Process Approach (HAPA) in the context of dietary change. In the social support literature, gender effects emerge with regard to the effectiveness of social support. Thus, a second aim was the examination of gender differences in the association of social support with dietary behavior. Methods: Participants were 252 overweight and obese individuals. At baseline and 12 months later, participants completed questionnaires on HAPA variables; diet-specific received social support and low-fat diet. Results: For the prediction of intentions 12 months later, instrumental support was more beneficial for men than for women over and above individual self-regulation. In terms of dietary behavior at T2, a moderate main effect of instrumental support emerged. Moreover, received emotional social support was beneficial for men, but not for women in terms of a low-fat diet 12 months later. Conclusions: Effects of received instrumental social support found in this study provide new evidence for the added value of integrating social support into the HAPA.