Objectives: To investigate the influence of internalized weight stigma (IWS) on physical activity (PA) outcomes among women with body mass index (BMI) over 30 kg/m(2). Design and method: Data were drawn from an RCT that included 80 primarily inactive women (94% nonHispanic/Latina white; mean age = 39.6, SD = 4.1, range = 30.0 to 45.0; mean BMI = 38.0 kg/m(2), SD = 3.9, range = 30.2-44.8 kg/m(2). Participants completed a 6 month weight-neutral, health-at-every-size or weight-loss-focused group-based healthy living program. PA enjoyment and engagement in moderate-intensity PA (MI-PA) (at least 30 minutes most days of the week) were assessed at baseline and immediately post-intervention. We used intention-to-treat linear mixed-effects modeling to test IWS as a moderator of changes in MI-PA engagement. We also tested a model whereby the positive effects of participating in the program on engagement in MI-PA would be serially mediated by a reduction in IWS and a concomitant increase in MI-PA enjoyment. Results: The weight-neutral and weight-loss-focused data were combined for all analyses. The moderation hypothesis was supported with a significant interaction between IWS and time. Participants had significant gains overall in MI-PA engagement from baseline to post-intervention; however, those with high IWS had an attenuated response. The serial mediation model was also supported. The positive effect of the program on engagement in MI-PA occurred through decreased IWS and increased MI-PA enjoyment. Conclusions: Self-directed stigma and holding negative attitudes about one's weight interferes with positive changes in PA outcomes. Healthy living programs may be less effective for those most vulnerable unless we aim to reduce IWS. (C) 2017 Elsevier Ltd. All rights reserved.