To examine the effect of a gender-tailored, Social Cognitive Theory (SCT)-based weight loss maintenance (WLM) intervention on men's physical activity and healthy eating cognitions and behaviours in the 12 months after completing a weight loss programme. A two-phase, assessor-blinded, randomized controlled trial. Ninety-two overweight/obese men (mean [SD] age: 49.2 years [10.1], BMI: 30.7 [3.3] kg/m(2) ) who lost at least 4 kg after completing the 3-month SCT-based SHED-IT Weight Loss Program were randomly allocated to receive (1) the SCT-based SHED-IT WLM Program; or (2) no additional resources (self-help control group). The 6-month gender-tailored SHED-IT WLM Program was completely self-administered and operationalized SCT behaviour change principles to assist men to increase moderate-to-vigorous physical activity (MVPA) and decrease energy-dense, nutrient-poor (discretionary) food consumption after initial weight loss. After randomization (WLM baseline), men were reassessed at 6 months (WLM post-test) and 12 months (6-month WLM follow-up). SCT cognitions (e.g., self-efficacy, goal setting), MVPA, and discretionary food consumption were assessed with validated measures. Following significant improvements in cognitions, MVPA and discretionary food consumption during the weight loss phase, intention-to-treat, linear mixed models revealed no significant group-by-time differences in cognitions or behaviours during the WLM phase. Initial improvements in MVPA and some cognitions (e.g., goal setting, planning, and social support) were largely maintained by both groups at the end of the study. Dietary effects were not as strongly maintained, with the intervention and control groups maintaining 57% and 75% of the Phase I improvements in discretionary food intake, respectively. An additional SCT-based WLM programme did not elicit further improvements over a self-help control in the cognitions or behaviours for MVPA or discretionary food intake of men who had lost weight with a SCT-based weight loss programme. Statement of contribution What is already known on this subject?Weight regain after weight loss (WL) is common. As successful weight loss maintenance (WLM) requires sustained improvements in cognitions and behaviours, health psychology can contribute to intervention development. However, little research has examined the utility of psychological theory in the context of a WLM randomised controlled trial. What does this study add? A theory-based WL program improved men's physical activity and dietary behaviours and cognitions. Men who also received a theory-based WLM program did not show further improvements in physical activity or dietary cognitions and behaviours compared to those that did not.