Purpose <p>The right-to-left ventricular blood pool T2 ratio (RV/LV T2 ratio) derived from cardiac magnetic resonance (CMR) T2 mapping is a potential biomarker of blood oxygenation. This study investigated the association between RV/LV T2 ratio and cognitive performance in heart failure (HF) secondary to ischemic heart disease (IHD).</p> Methods <p>This retrospective study included 52 patients with chronic HF and 26 healthy controls, all of whom underwent CMR and neuropsychological testing. Regions of interest were manually drawn in RV and LV blood pools to calculate RV/LV T2 ratio. Cognitive performance was assessed by the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA). Group comparisons and linear, multivariate, and mediation analyses were performed.</p> Results <p>HC demonstrated higher RV/LV T2 ratio than patients with HF with preserved or reduced and mildly reduced ejection fraction (<i>p</i> = 0.012, and <i>p</i> &lt; 0.001). Patients with cognitive impairment (MoCA-defined) showed lower RV/LV T2 ratios than cognitively normal patients (<i>p</i> = 0.010). RV/LV T2 ratio was positively related to MMSE (β = 0.336, <i>p</i> = 0.020) and MoCA score (β = 0.563, <i>p</i> &lt; 0.001). In multivariate linear regression analysis, RV/LV T2 ratio was the only CMR predictor for MoCA score (β = 0.340, <i>p</i> = 0.015). Mediation analysis showed RV/LV T2 ratio partially mediated the correlation between stroke volume and MoCA score, with a mediation effect ratio of 44.3%.</p> Conclusion <p>RV/LV T2 ratio is an additional CMR biomarker to evaluate the cognitive performance in patients with HF secondary to IHD.</p>