BACKGROUND: This paper describes and discusses the transition of and modifications to a weight management randomized controlled trial among active-duty military personnel from an in-person to a virtual format as a result of the COVID-19 pandemic. The original pragmatic cohort-randomized controlled trial was designed to compare the effectiveness of an 8-week group weight management program, ShipShape, to a version of ShipShape enhanced with acceptance and commitment therapy. OBJECTIVE: The objective of our study was to assess potential differences between in-person and virtual participation in participants' demographics, motivation, confidence, credibility, expectations, and satisfaction with the interventions; we also examined the pragmatics of the technology and participants' experiences in virtual-format intervention groups. METHODS: A total of 178 active-duty personnel who had failed or were at risk of failing their physical fitness assessment or were overweight or obese were enrolled in the study. In-person (n=149) and virtual (n=29) participants reported demographics, motivation, confidence, credibility, expectations, and satisfaction. Interventionists recorded attendance and participation in the group sessions. Independent-sample 2-tailed t tests and chi-square tests were used to compare the characteristics of the in-person and virtual participants. Pragmatics of the technology and participants' experiences in the virtual format were assessed through surveys and open-ended questions. RESULTS: Participants were 29.7 (SD 6.9) years old on average, 61.8% (110/178) female, and 59.6% (106/178) White and had an average BMI of 33.1 (SD 3.9) kg/m2. Participants were highly motivated to participate and confident in their ability to complete a weight management program. A total of 82.6% (147/178) of all participants attended 5 of the 8 sessions, and participation was rated as "excellent" by interventionists in both formats. The interventions were found to be credible and to have adequate expectations for effectiveness and high satisfaction in both formats. There were no differences between in-person and virtual participants in any of these metrics, other than interventionist-rated participation, for which virtual participants had significantly higher ratings (P<.001). Technical satisfaction with the virtual sessions was rated as "good" to "very good," and participants were satisfied with the content of the virtual sessions. A word cloud of responses identified "mindfulness," "helpful," "different," "food," "binder," and "class" as concepts the virtual participants found most useful about the program. CONCLUSIONS: Modifications made in response to the COVID-19 pandemic were successful, given the recruitment of active-duty personnel with similar demographic characteristics, attendance levels, and indicators of credibility, expectancy, and satisfaction in the virtual format and the in-person format. This successful transition provides support for the use of virtual or digital weight management interventions to increase accessibility and reach among highly mobile active-duty personnel. TRIAL REGISTRATION: ClinicalTrials.gov NCT03029507; https://clinicaltrials.gov/ct2/show/NCT03029507.