This study examined the potential of left atrial (LA) reservoir strain to be an independent determinant of right ventricular (RV) dysfunction (RVD) in the context of left-sided heart failure (HF). We recruited 301 patients with HF (age 61.1±10.7 years; 71.1% male) from The First Hospital of China Medical University between 2021 and 2023, comprising a representative sample of both reduced (47.8%) and preserved ejection fraction (EF) (52.2%) phenotypes. Each participant underwent comprehensive transthoracic echocardiography. Conventional echocardiography measured left ventricular (LV) EF and pulmonary artery systolic pressure (PASP), and two-dimensional speckle tracking echocardiography was applied to quantify LV global longitudinal strain (LVGLS), LA reservoir strain, and RV free wall strain. The cohort demonstrated a 48.5% prevalence of RVD (defined as RV free wall strain < |20%|). RVD was associated with higher body surface area [odds ratio (OR) 12.01] and PASP (OR 1.02), lower LVEF (OR 0.92), LVGLS (OR 1.35), and LA reservoir strain (OR 0.82) (all P values <0.05). Critically, LA reservoir strain (OR 0.83; P<0.001) remained independently associated with RVD even after adjustments were made for age, sex, body surface area, atrial fibrillation, diabetes, hypertension, coronary artery disease, LVEF, LVGLS, and PASP. Thus, in left-sided HF, LA reservoir strain independently predicts RVD irrespective of LVEF or PASP, highlighting the critical role of LA function in RVD.