Abstract Background Postmenopausal smokers have difficulty quitting smoking and experience considerable weight gain with smoking cessation. We examined whether adjunctive smoking treatment with exercise, compared to a relaxation control condition, could improve cigarette abstinence, decrease cigarettes smoked per day (CPD), and ameliorate changes in body mass index (BMI) in postmenopausal smokers. Methods Women (N = 301) signed informed consent and were randomized to treatment at two sites (Universities of Connecticut and Minnesota). We randomized groups of participants to a comprehensive group treatment program that included 12 weeks of varenicline and either a moderate exercise or relaxation component for 6 months. Participants were followed for a year after medication treatment. Results Overall, 17.3% of patients reported carbon monoxide-verified continuous abstinence for the 9- to 12-week period, and 11.6% reported prolonged abstinence at 1 year, with no significant differences between treatment conditions. CPD reported at study visits showed significant main effects for time in weeks, for site, and for treatment. The Exercise condition reported smoking fewer CPD over time, and that advantage widened over time. In terms of BMI, significant effects for time in weeks, and for the interaction of Week × Treatment condition, reflected gradually increasing BMI in these women over time, but with the increase in BMI slower in the Exercise condition. Conclusions Exercise, compared to relaxation, was associated with a reduced BMI and CPD in postmenopausal women, but did not increase end of treatment or prolonged abstinence. Further research is needed to devise exercise programs that increase smoking cessation rates in postmenopausal women. Implications This study adds to the literature on the effectiveness of a moderate exercise intervention compared to a relaxation control condition as an adjunctive treatment for smoking cessation in postmenopausal women. Our exercise program did not increase end of treatment or prolonged abstinence rates in postmenopausal women; however, there was a beneficial effect on smoking reduction and reduced body mass index. Additional research is needed to devise exercise programs that increase smoking cessation rates in postmenopausal women.