BACKGROUND: There is considerable interest to develop accurate subject-specific biomechanical models of the knee. Most of the existing models currently do not include a representation of the posterior knee capsule. In order to incorporate the posterior capsule in knee models, data is needed on its mechanical properties. OBJECTIVE: To quantify the mechanical properties of the human posterior knee capsule through semi-static tensile tests. METHODS: Fifteen posterior knee capsule specimens (5 knees, 3 male, 2 female; age 79.2±7.9 years) were used to perform tensile tests. A medial, central and lateral specimen was taken from each knee. The cross-sectional area was measured, after which semi-static tensile tests were performed to quantify the material properties. RESULTS: The stiffness of the capsule was randomly distributed over the regions. The global Young's modulus and yield strength was 8.58±10.77 MPa and 1.75±1.89 MPa, respectively. A strong correlation (ρ=0.900) was found between Young's modulus and yield strength. The location of failure was not associated with smallest cross-sectional area or highest strain. CONCLUSIONS: The results suggest that the posterior knee capsule does not have a systematic (medial–central–lateral) distribution of material properties. The posterior capsule may play an important role in knee joint mechanics, particularly when in hyper extension.