Obesity is common among critically ill adults, but its association with outcomes during venovenous extracorporeal membrane oxygenation (VV ECMO) remains controversial. We evaluated associations of body mass index (BMI) with mortality and Extracorporeal Life Support Organization (ELSO)–defined complications in adults supported with VV ECMO. We conducted a retrospective multicenter cohort study of adults receiving VV ECMO at four centers (2017–2025). The primary outcome was in-hospital mortality; secondary outcomes were ELSO-defined complications. Multivariable logistic regression was adjusted for age, sex, and hospital site; BMI was evaluated as continuous, dichotomized (≥30 vs. <30 kg/m²), and World Health Organization (WHO) obesity class exposures, with interaction testing by coronavirus disease 2019 (COVID-19) status. Among 420 patients (median BMI, 31 kg/m²), in-hospital mortality was 62%. Higher BMI was associated with mortality in non–COVID-19 acute respiratory distress syndrome (ARDS; odds ratio [OR] = 1.06 per 1 kg/m², 95% confidence interval [CI] = 1.02–1.10, p = 0.003) but not in COVID-19 ARDS (OR = 1.00, 95% CI = 0.94–1.05); there was insufficient evidence of interaction between BMI and COVID-19 (p = 0.07). Higher BMI was associated with renal complications in both groups and infectious complications in non–COVID-19 ARDS (OR = 1.05, 95% CI = 1.02–1.09, p = 0.003). No significant BMI × COVID-19 interaction was detected for any outcomes. These findings do not support an obesity survival paradox during VV ECMO.