The use of goniometers to measure joint angles is a key part of musculoskeletal practice. Recently smartphone goniometry applications have become available to clinicians.This study examined the intra- and inter-measurer reliability of novice and experienced clinicians and the concurrent validity of assessing knee range of motion using a smartphone application (the Knee Goniometer App (Ockendon)) (KGA) and a standard universal goniometer (UG).Three clinicians, each with over seven years' experience as musculoskeletal physiotherapists and three final year physiotherapy students, measured 18 different knee joint angles three times, using both the universal goniometer and the smartphone goniometric application.The universal goniometer and the smartphone goniometric application were reliable in repeated measures of knee flexion angles (average Concordance Correlation Coefficient (CCC)>0.98) with both experienced clinicians and final year physiotherapy students (average CCCs>0.96). There were no significant differences in reliability between the experienced and the novice practitioners for either device. Agreement between the universal goniometer and smartphone goniometric application measurements was also high for all examiners with average CCCs all above 0.96. The Standard Error of Measurement ranged between 1.56° (0.52-2.66) for the UG and 0.62° (0.29-1.27) for the KGA.The universal goniometer and the smartphone goniometric application were reliable in repeated measures of knee flexion angles. Smaller error of measurement values for the smartphone goniometric application might indicate superiority for assessment where clinical situations demand greater precision of knee range of motion.