Goniometry is a commonly used method for the clinical assessment of range of motion (ROM) of the knee. A digital photograph of the knee in maximal flexion and extension could provide a more objective way to assess the ROM. The aim of our study was to investigate the reliability of the use of digital photographs as a method for measuring the ROM of the knee. Four observers examined 49 patients. Digital photographs of the knee in maximal flexion and extension were compared with standard clinical goniometric measurements in the same position. We observed higher intra-observer reliability for the digital method in flexion (p < 0.0001) and extension (p = 0.005) compared to goniometry. The extension results were numerically lower when using goniometry compared to the digital method (p < 0.001). For both methods, the intra-observer reliability for extension was lower compared to flexion. The intraobserver standard error of measurement (SEM) of the digital method was smaller than the goniometric SEM; the digital method thus appeared more reliable. Measuring maximal flexion and extension on digital photographs is more reliable for both extension and flexion compared to standard goniometric measurements.