ABSTRACT Background Being overweight is associated not only with physical health problems, but also with risk of mental health problems. Increased physical activity (PA) has been recommended for the prevention of cardiovascular disease; however, little is known about the effect of walking on physical and mental health outcomes. Objective The purpose of the study was to explore the effectiveness of a pedometer-based PA intervention on physical and mental health states. Method Thirty-five overweight participants with body mass index (BMI) ≥25 kg•m–2 were selected and assigned to a 12-week pedometer-based walking program (10,000 steps•d–1). The profile of mood states, BMI, waist circumference (WC), body fat percentage (%BF), and lean body mass (LBM) were measured before and after the 12-week intervention. The number of step counts was recorded 5 days a week in a diary booklet. Results The 30 participants who accumulated 10,000 steps•d–1 had significantly lower anxiety, depression, anger, fatigue, confusion, and total mood distress scores compared with measurements taken prior to the intervention. Further, the participants had higher vigor scores compared to baseline. Regarding physical health, the participants who accrued 10,000 steps a day had significantly lower body weight, WC, BMI, and %BP. After adjustment for gender, height, and daily steps at follow-up, changes in WC were negatively associated with depression, fatigue, confusion, and total mood distress. Conclusions An increase in PA by accumulating at least 10,000 steps•d–1 over a 12-week period improves physical and mood states in sedentary, overweight individuals.