Background: The health benefits of regular physical activity are well known and include the prevention of chronic diseases such as obesity, type 2 diabetes, and cardiovascular diseases. Still, only 20% of U.S. adults report meeting the Physical Activity Guidelines for Americans. With approximately 43% of U.S. jobs considered sedentary, there is a need for effective workplace physical activity interventions. MapTrek is a mobile health game designed to increase daily physical activity in a low-cost, scalable, and enjoyable way. Objective: The purpose of the present study was to test the efficacy of MapTrek for increasing daily steps and moderate-intensity steps over 10 weeks in a sample of sedentary office workers. Methods: Participants included 144 full-time sedentary office workers ages 21-65 who reported sitting at least 75% of their workday. Each participant received a Fitbit Zip to wear daily throughout the intervention. Participants were randomized to either a: 1) Fitbit only group or 2) Fitbit + MapTrek group. Physical activity outcomes and intervention compliance were measured with the Fitbit activity monitor. Results: The Fitbit + MapTrek group significantly increased daily steps (+2,091.5 steps/day) and active minutes (+11.2 minutes/day) compared to the Fitbit only arm. Conclusions: These data support MapTrek as an effective approach for increasing physical activity at a clinically meaningful level in sedentary office workers.