•Investigated a body image program for women who have undergone bariatric surgery.•Experimental and wait-list groups experienced quantitative improvements in body image.•Qualitative analyses showed the experimental group experienced positive body image.•Qualitative analyses also revealed challenges to women’s body image. This study investigated a novel technique to improve body image among women who have undergone bariatric surgery—namely, by having them focus on their body functionality (everything the body can do, rather than how it looks). Participants were 103 women (Mage = 46.61) who had undergone bariatric surgery 5–7 months prior to the study. They were randomized to the 1-week online intervention, comprising three functionality-focused writing exercises (Expand Your Horizon; Alleva et al., 2015), or to a wait-list control group. Body appreciation, appearance and functionality satisfaction, body awareness, self-objectification, self-esteem, and self-kindness were assessed at pretest, posttest, and at 1-week and 3-month follow-up. Multilevel modeling analyses showed that, compared to the control, the intervention group experienced improved body appreciation at posttest, and these improvements persisted at both follow-ups. These findings were nonsignificant when intent-to-treat analyses were performed. Both available case and intent-to-treat analyses showed that all participants experienced improvements in facets of body image across time. Qualitative analyses of participants’ responses to the intervention writing exercises provided more insight. Via coding reliability thematic analysis, we identified 11 themes that together provide evidence that intervention participants experienced facets of a more positive body image, while also facing challenges to their body image and well-being. Together, findings suggest that focusing on body functionality may contribute to improved body image among women who have undergone bariatric surgery, but effects may be nuanced compared to prior functionality research among general samples of women. The study was registered retrospectively (ClinicalTrials.gov; identifier NCT04883268).