IMPORTANCE: Excess body weight is a strong risk factor for atrial fibrillation (AF), and weight loss is recommended in clinical guidelines for all patients with obesity and AF. However, existing evidence derives from younger patients, and weight loss in older adults could precipitate frailty. OBJECTIVE: To investigate whether weight loss is a safe and effective intervention in older patients with overweight and AF. DESIGN, SETTING, AND PARTICIPANTS: Parallel-group, unblinded, randomized clinical trial conducted at 2 UK hospitals from November 14, 2018, to April 25, 2025. Approximately 1500 individuals undergoing electrical cardioversion for AF were assessed for eligibility, and approximately 500 aged 60 to 85 years with body mass index 27 or greater and without any exclusion criteria were invited to participate. Of these, 119 provided informed consent to enter the trial and 118 were randomized. INTERVENTION: Participants were randomized to an 8-month low-calorie diet and behavioral support program (intervention, n = 59) or to usual care (control, n = 59). MAIN OUTCOME AND MEASURE: Intention-to-treat analysis of the change in Atrial Fibrillation Severity Scale (AFSS) symptom severity score at 8 months after randomization. RESULTS: Participants (n = 118) had a mean age of 68 years (SD, 6); 33% were female. The intervention resulted in significantly lower weight (baseline-adjusted mean weight at 8 months: 92.6 [SE, 0.85] kg vs 99.4 [SE, 0.85] kg; P < .001; estimated difference, -6.9 kg [95% CI, -9.2 to -4.5]). This corresponded to a weight reduction of 9.7% vs 3.1%, respectively (P < .001). However, there were no significant differences between the groups in AFSS symptom severity score (baseline-adjusted mean at 8 months, 7.9 [SE, 0.84] in the intervention group vs 8.9 [SE, 0.84] in the control group; between-group difference, -0.9 [95% CI, -3.3 to 1.4]; P = .43). No significant treatment effects were observed on physical performance, AF burden, cardiac imaging parameters, blood pressure, lipid profile, or incidence of repeat cardioversion or AF ablation during follow-up. No serious adverse events related to participation in the trial were reported in either group. CONCLUSIONS AND RELEVANCE: In older patients with overweight and persistent AF, a low-calorie diet and behavioral support program was associated with significant weight loss at 8 months with no safety concerns but did not affect AF symptoms, AF burden, cardiac remodeling, or the need for further rhythm control interventions. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03713775.