ImportanceEffective weight loss interventions are needed for men with obesity.ObjectiveTo determine whether an intervention that combined text messaging with financial incentives attained significant weight loss at the 12-month follow-up compared with the control group and whether an intervention of text messaging alone attained significant weight loss at the 12-month follow-up compared with the control group.Design, Setting, and ParticipantsAn assessor-blinded randomized clinical trial conducted in Belfast, Bristol, and Glasgow areas in the UK. A total of 585 men with body mass index (BMI) of 30 or more were enrolled between July 2021 and May 2022. Final follow-up occurred June 2023.InterventionsParticipants were randomly assigned to 12 months of behavioral focused text messages combined with financial incentives (n = 196), 12 months of behavioral focused text messages alone (n= 194), or a waiting list (control group; n= 195). The financial incentive consisted of a monetary reward that was lost if weight loss targets were not met. All participants received weight management information and a pedometer at baseline.Main Outcomes and MeasuresThe 2 primary comparisons were the 12-month comparison of within-participant weight change between the text messaging with financial incentive group and the control group and the comparison between the text messaging alone group and the control group (minimum clinically important difference, 3%). The P value defined for statistical significance was P < .025 for each comparison.ResultsOf the 585 men (mean [SD] age, 50.7 [13.3] years; mean weight, 118.5 [19.9] kg; mean BMI, 37.7 [5.7]; 525 [90%] White), 227 (39%) lived in postal code areas with lower socioeconomic status, and 426 (73%) completed the 12-month follow-up. At the 12-month follow-up, compared with the control group, the mean percent weight change was significantly greater in the text messaging with financial incentive group (mean difference, −3.2%; 97.5% CI, −4.6% to −1.9%; P < .001) but was not significantly greater in the text messaging alone group (mean difference, −1.4%; 97.5% CI, −2.9% to 0.0, P = .05). The mean (SD) weight changes were −5.7 (7.4) kg for the text messaging with financial incentives group, −3.0 (7.5) kg for the text messaging alone group, and −1.5 (6.6) kg for the control group. The 12-month mean (SD) percentage weight changes from baseline were −4.8% (6.1%) for the text messaging with financial incentives group, −2.7% (6.3%) for text messaging alone group, and −1.3% (5.5%) for the control group. Of 366 adverse events reported, the most common were infections (83 [23%]). Of the 23 serious adverse events (6.3%), 12 (52%) occurred in the text messaging with financial incentives group, 5 (22%) in the texts messaging alone group, and 6 (26%) in the control group. None were considered related to participating in a trial group.Conclusion and RelevanceAmong men with obesity, an intervention with text messaging with financial incentive significantly improved weight loss compared with a control group, whereas text messaging alone was not significantly better than the control condition. These findings support text messaging combined with financial incentives to attain weight loss in men with obesity.Trial Registrationisrctn.org Identifier: ISRCTN91974895