Abstract Adults attempting weight loss through lifestyle modification (LM) typically find maintenance of behavior change difficult. Outcomes might be improved if participants are provided with sustained sources of accountability and support and ongoing opportunities to reflect with others on goal progress. This study proposes that sharing digital data (i.e., body weight from wireless scale, physical activity from wearable sensor, and dietary intake from smartphone app) with other parties has the potential to improve long-term weight loss. The benefit of device data sharing has not yet been rigorously tested, and traditional LM programs do not yet incorporate digital data sharing in a systematic way. The proposed study will enroll adults (N = 320) with overweight/ obesity in a 24-month LM program and instruct them to use digital tools for self-monitoring of weight, physical activity, and eating on a daily basis. Groups will meet face-to-face weekly in months 1-3 to initiate weight loss. In months 4-24, intervention contact will be remote and will include the following: quarterly group meetings held via videoconference; brief phone calls with the coach held twice per quarter; and monthly text messages with the coach, with a small group of fellow group participants, and with a friend or family member outside of the program. A 2 x 2 x 2 factorial design will test the independent effects of three types of data sharing partnerships: Coach Share, Group Share, and Friend/Family Share. Half of the participants will receive Coach Share and half will not; half will receive Group Share and half will not; and half will receive Friend/Family Share and half will not. In Coach Share, the behavioral coach will view digital self-monitoring data throughout the program and will directly address data observations during intervention contacts. In Group Share, participants in a given LM group will view each other’s self-monitoring data in their small-group text messages. In Friend/Family Share, a friend or family member outside of the group will view the participant’s data via automated text message. Each party with whom data are shared will be trained to respond by eliciting reflection from the index participant on his/her goal progress, which is a key component of self-regulation, and supporting the participant’s motivation to meet program goals. Amount of intervention contact between the participant and each party (Coach, Group, Friend/Family) will be comparable across treatment conditions, isolating the effects of data sharing components. Outcomes will be measured at months 0, 6, 12, and 24. The study will determine if Coach Share, Group Share, and Friend/Family Share each improve long-term weight loss, PA, and calorie intake (i.e., outcomes will be compared for participants who are randomized to engage in that data sharing partnership, versus those who are not). The study also will examine if effects are additive when participants are assigned to engage in more th...