(1) Clinical alternatives to the full-leg radiograph (FLR) for determining frontal plane lower limb alignment have shown mixed results so far concerning reliability and accuracy. In addition, intercondylar respectively intermalleolar distance (ICD/IMD) measurement is limited in the interpretation of its results. The purpose of the study is to present a standardized, sonographic-assisted photographic measurement (SPM) of the leg axis in the frontal plane using a digital photograph and compare it with the gold standard of the anterior-posterior FLR. (2) Adults who had recently received an FLR were included in the study. After determining the center of the hip joint and knee joint gap in a standing position via ultrasound, a standardized digital photograph was taken. Subsequently, the hip-knee-ankle angle (HKA) was measured using an image editing program for SPM and FLRs. Mean deviation (MD), Pearson's correlation coefficient and the clinical method's interclass correlation coefficient (ICC) were calculated. (3) Of 18 subjects (8 male and 10 female), respectively, 34 lower extremities have been included in the study. Their mean BMI was 25.5 kg/m(2). The correlation between the two measurement methods for subjects with BMI < 30 kg/m(2) was reliably high (r = 0.93, p < 0.001). The HKAs derived from SPM and FLRs showed a mean deviation (MD) of 2.4 degrees (range 0.1-6.0 degrees) for subjects with BMI < 30 kg/m(2). The average ICC among all examiners conducting the SPM was 0.99 (CI 0.95-0.99, p < 0.001). The MD for subjects = 30 kg/m(2) increased significantly (MD = 5.5 degrees; n = 10, p < 0.001). (4) SPM of the leg axis in the frontal plane allows sufficiently accurate results in patients with a BMI lower than 30 kg/m(2). The methodology is limited in cases of increased BMI and needs well-trained examiners. Nevertheless, as a screening method in the field of children and adolescents, this method can be recommended in clinical daily routine.