Purpose: Hereditary connective tissue disorders (HCTDs), such as classic Ehlers–Danlos syndrome (cEDS) and Marfan syndrome (MS) share overlapping features like hypermobility and tissue fragility. In clinical practice it remains a challenge to distinguish children and adolescents with HCTD from healthy children. The purpose of this study was to investigate the biomechanical properties of the patellar tendon and joint laxity (Beighton score) in children with HCTDs (n = 7) compared to healthy controls (n = 14). Methods: The mechanical properties of the patellar tendon were assessed using simultaneous force and ultrasonographic measurements during isometric ramp contractions. Ultrasonography was also used to measure tendon dimensions. The HCTD children were matched with 2 healthy controls with regard to age, body mass index (BMI), sex and physical activity level. Results: The HCTD children had a greater degree of joint laxity (P < 0.01). Although, the patellar tendon dimensions did not differ significantly between the two groups, the HCTD children showed a tendency toward a larger patellar tendon cross-sectional area (CSA) (35%, P = 0.19). Moreover, stiffness did not differ between the two groups, but secant modulus was 27% lower in children with a HCTD (P = 0.05) at common force and 34% lower at maximum force (P = 0.02). Conclusions: The present study demonstrates for the first time that children with HCTDs have lower material properties (modulus) of their patellar tendon, which may be indicative of general impairment of connective tissue mechanics related to their increased joint laxity.