Aims Late systolic dysfunction of the systemic right ventricle (RV) in patients with transposition of the great arteries (TGA) is of major concern. Right ventricular global longitudinal strain (GLS) might be able to identify early dysfunction. Methods and results Adults with TGA after Mustard operation (TGA-Mustard) or congenitally corrected-TGA (ccTGA) underwent echocardiography, electrocardiography, and NT-proBNP measurement. Using speckle-tracking echocardiography, we analysed longitudinal strain and strain rate, and compared findings in both patients groups, to healthy controls and with clinical parameters. We included 42 patients (mean age 37 +/- 7 years, 69% male) with a systemic RV [32 TGA-Mustard (34 +/- 4 years after corrective surgery) and 10 ccTGA], and 32 healthy controls (mean age 36 +/- 11 years). Global longitudinal strain of the systemic RV was lower in patients than GLS of the systemic LV in controls (-14.2 +/- 3.5 vs. -20.0 +/- 3.0%, P < 0.001). Average LS of the RV lateral wall was lower in patients with TGA-Mustard (-15.5 +/- 3.4%) than ccTGA (-18.3 +/- 3.6%, P = 0.047). Right ventricular GLS tended to be lower in patients in NYHA class II than I, and correlated with NT-proBNP (r = 0.49, P = 0.001), RV fractional area change (r = -0.39, P = 0.019), RV apex-base-diameter (r = 0.37, P = 0.021), and QRS duration (r = 0.41, P = 0.014). Conclusion Global longitudinal strain of the systemic RV in patients is lower than GLS of the systemic LVin healthy controls, especially in the apical segment, and tended to be lower in TGA-Mustard than ccTGA patients. Since RV GLS correlates with RV function, myocardial deformation is useful as a more quantitative tool to measure systemic RV function. Decreased GLS was associated with elevated NT-proBNP and tended to correlate with worsening NYHA class, which strengthens the potential prognostic value of GLS in patients with a systemic RV.