BACKGROUND: Chronic thromboembolic pulmonary hypertension (CTEPH) leads to right atrial (RA) dysfunction, which correlates with poor prognosis. However, most studies on RA function in pulmonary hypertension (PH) focus on heterogeneous PH etiologies, with limited data on CTEPH alone. This study aimed to assess RA function in CTEPH patients using standard two-dimensional (2DE) and M-mode echocardiography, and validate the clinical value of RA-related parameters. METHODS: We enrolled 91 CTEPH patients and 30 healthy controls. RA volume/function parameters (maximal volume index [RAVmaxI], total/passive/active emptying fractions [TotEF/PassEF/ActEF]) and tricuspid annular plane systolic excursion (TAPSE, decomposed into atrial [TAPSEra] and ventricular [TAPSErv] components; TAPSEra% = TAPSEra/TAPSE) were measured via 2DE/M-mode echocardiography. Correlations with clinical (WHO functional class [WHO-FC], 6-min walk distance [6MWD]) and laboratory (NT-proBNP) indices were analyzed; receiver operating characteristic (ROC) curves evaluated predictive value for WHO-FC ≥ III. RESULTS: Compared to controls, CTEPH patients had higher RAVmaxI (43.46 ± 13.34 vs. 22.52 ± 2.89 mL/m2, P < 0.001), lower TotEF (39.45 ± 9.43 vs. 50.07 ± 7.52%, P < 0.001) and PassEF (14.33 ± 6.43 vs. 30.03 ± 5.26%, P < 0.001), and higher ActEV/TotEV (59.76 ± 17.37 vs. 34.05 ± 12.75%, P < 0.001). TAPSEra% was higher in CTEPH patients (58.69 ± 19.54 vs. 30.52 ± 7.92%, P < 0.001). RAVmaxI (≥37.47 mL/m2, AUC = 0.899, sensitivity = 75.4%, specificity = 91.7%) and TAPSEra% (≥45.05%, AUC = 0.849, sensitivity = 90.2%, specificity = 70.0%) effectively predicted WHO-FC ≥ III (both P < 0.001). CONCLUSIONS: Impaired RA reservoir and conduit functions are hallmarks of CTEPH, with compensatory active contraction counteracting the reduction in passive filling. Given their noninvasive nature and high reliability, RAVmaxI and TAPSEra% are valuable indices for identifying patients with WHO-FC ≥ III and quantifying CTEPH severity, justifying their integration into standard echocardiographic protocols.