Abstract Objective To evaluate the marginal costs of increasing physical activity and maintaining weight for a lifestyle physical activity program targeting sedentary African American women. Methods Outcomes included change in minutes of total moderate to vigorous physical activity, leisure-time moderate to vigorous physical activity and walking per week, and weight stability between baseline and maintenance at 48 weeks. Marginal cost-effectiveness ratios (MCERs) were calculated for each outcome, and 95% confidence intervals (CIs) were computed using a bootstrap method. The analysis was carried out from the societal perspective and calculated in 2013 US dollars. Results For the 260 participants in the analysis, program costs were $165 ± $19, and participant costs were $164 ± $35, for a total cost of $329 ± $49. The MCER for change in walking was $1.50/min/wk (95% CI 1.28–1.87), for change in moderate to vigorous physical activity was $1.73/min/wk (95% CI 1.41–2.18), and for leisure-time moderate to vigorous physical activity was $1.94/min/wk (95% CI 1.58–2.40). The MCER for steps based on the accelerometer was $0.46 per step (95% CI 0.30-0.85) and weight stability was $412 (95% CI 399–456). Conclusions The Women’s Lifestyle Physical Activity Program is a relatively low-cost strategy for increasing physical activity. The marginal cost of increasing physical activity is lower than for weight stability. The participant costs related to time in the program were nearly half the total costs, suggesting that practitioners and policymakers should consider the participant cost when disseminating a lifestyle physical activity program into practice.