Many adults are considered sedentary, a lifestyle that is often detrimental to overall health. Monetary incentive interventions have been successful in increasing physical activity in underactive and sedentary adults but are often costly. Deposit contracts are arrangements in which a participant deposits a specified amount of money, which he or she can earn back for meeting behavioral goals. In the current ABA design study, adults who walked fewer than 10,000 steps during a baseline period were randomly assigned to a deposit or no-deposit group. Both groups could earn up to $50 for meeting step count goals during a 21-day intervention period. Only those in the deposit contract group were required to contribute $25. Goals were individualized and calculated by using percentile schedules. The final week was a return-to-baseline condition. Fourteen of 19 participants increased their average daily step counts by at least 2,500 steps during the intervention period compared to the initial baseline. A significant effect of the intervention was detected, and no differences were observed between the deposit and no-deposit groups. These findings support the use of deposit contracts in physical activity interventions.