Abstract: Objective: To investigate the impact of obesity on cardiac reverse remodeling in patients with heart failure with reduced or mildly reduced ejection fraction (HFrEF/HFmrEF) receiving guideline-directed medical therapy (GDMT). Methods: A retrospective analysis was conducted on patients with a left ventricular ejection fraction (LVEF) <50% treated at the First Affiliated Hospital of Nanjing Medical University between January 2022 and October 2024. All patients received GDMT and underwent baseline cardiac magnetic resonance (CMR) imaging to assess myocardial fibrosis using late gadolinium enhancement (LGE) and extracellular volume fraction (ECV). Obesity was defined as a body mass index (BMI) ≥28 kg/m2. Cardiac reverse remodeling was evaluated by echocardiography at baseline and after 6 months of GDMT. Treatment response was defined as an absolute increase in LVEF ≥5%. Results: Among the 80 enrolled patients (31 obese, 49 non-obese), the obese group showed a smaller improvement in LVEF and a lower treatment response rate compared to the non-obese group (ΔLVEF: 14.45% ± 2.08% vs. 20.80% ± 1.81%, P=0.024; response rate: 76.80% vs. 93.58%, P=0.036). Multivariable regression analysis confirmed that BMI was an independent negative predictor of LVEF improvement (β=-0.90, P=0.004). Both LGE (OR=0.68, P=0.025) and ECV (OR=0.89, P=0.012) were independent predictors of reduced treatment response. A predictive model combining BMI and ECV demonstrated the highest accuracy for identifying poor responders (AUC=0.923), which was significantly superior to the baseline model (P=0.043). Conclusion: In HFrEF/HFmrEF patients, obesity is associated with attenuated cardiac reverse remodeling and reduced responsiveness to GDMT. The combination of BMI and ECV improves the identification of patients at risk for diminished treatment response.