Background: The right ventricle serves as the subaortic systemic ventricle (sysRV) in patients with congenitally corrected transposition of the great arteries (ccTGA) and in patients with transposition of the great arteries (TGA) surgically repaired by an atrial switch. SysRV can lead to late complications, primarily heart failure, significant regurgitation of the systemic atrioventricular (AV) valve, and ventricular arrhythmias with sudden cardiac death. We sought to investigate the value of 2D- and 3D-echocardiographic parameters of sysRV function. Methods: Consecutive adult patients with sysRV who presented at the adult congenital heart disease outpatient clinic were prospectively enrolled. All patients received comprehensive transthoracic echocardiography, including 3D-echocardiography, cardiac magnetic-resonance-imaging (CMR), cardiopulmonary-exercise-testing, and blood analysis for NT-proBNP. Results. A total of 27 patients were included, 18 with TGA and nine with ccTGA. Median age was 37 years (Q1 = 31, Q3 = 44), 44% were male, median NT-proBNP was 189 pg/mL (Q1 = 155, Q3 = 467); sufficient 3D-echocardiography datasets were acquired in 78% of patients. All echocardiographic 2D and 3D volumetric function parameters correlated with CMR data, whereas a correlation was not seen with any of the longitudinal function parameters. NT-proBNP correlated with tricuspid annular plane systolic excursion (r = −0.43, p = 0.02) and CMR ejection fraction (EF) (r = −0.62, p = 0.003). Conclusion: Systematic evaluation of sysRV is complex and should include not only volumetric parameters but also parameters of longitudinal function in addition to measurement of NT-proBNP. In patients with good image quality, 3D-echocardiography can be used to assess volumes and EF.