Purpose The purpose of this paper is to test the effects of augmenting an evidence-based physical activity intervention within an existing commercial weight loss program to assess effects on increasing physical activity and reducing psychological distress. Design/methodology/approach The CONSORT guidelines were adopted for the study. In total, 49 women with overweight or obesity (M age=39.5, SD:12.4; M Body Mass Index=31.02, SD: 2.10) enrolled in a six week commercial weight loss program were randomized to an intervention or a control group. Participants in the control group received care as usual; participants in the intervention group additionally received an evidence-based intervention to increase physical activity that included behavior change techniques including implementation intentions, goal-setting and self-monitoring. Findings Weekly steps increased in the intervention group (M=31,516.25; SD=9,310.17 to M=62,851.36; SD=13,840.4) significantly more (p<0.001, η p 2=0.32) than in the control group (M=30,207.67; SD=7,833.29) to M=46,969.33 (SD=9,470.96), along with experiencing significantly lower anxiety (p<0.001, η p 2=0.15), social dysfunction (p<0.001, η p 2=0.16) and depression symptoms (p<0.05, η p 2=0.08) at follow-up. Research limitations/implications This intervention warrants extension to those seeking to improve mental health through physical activity. Originality/value This study took a novel approach of augmenting a commercial weight loss program with a theory-based physical activity module, showing positive effects for physical activity behavior and psychological health.