The aim of this study was to assess the reliability of a smartphone-based application developed for photographic-based goniometry, DrGoniometer (DrG), by comparing its measurement of the knee joint angle with that made by a universal goniometer (UG). Joint goniometry is a common mode of clinical assessment used in many disciplines, in particular in rehabilitation. One validated method is photographic-based goniometry, but the procedure is usually complex: the image has to be downloaded from the camera to a computer and then edited using dedicated software. This disadvantage may be overcome by the new generation of mobile phones (smartphones) that have computer-like functionality and an integrated digital camera. This validation study was carried out under two different controlled conditions: (i) with the participant to measure in a fixed position and (ii) with a battery of pictures to assess. In the first part, four raters performed repeated measurements with DrG and UG at different knee joint angles. Then, 10 other raters measured the knee at different flexion angles ranging 20-145° on a battery of 35 pictures taken in a clinical setting. The results showed that inter-rater and intra-rater correlations were always more than 0.958. Agreement with the UG showed a width of 18.2° [95% limits of agreement (LoA)=-7.5/+10.7°] and 14.1° (LoA=-6.6/+7.5°). In conclusion, DrG seems to be a reliable method for measuring knee joint angle. This mHealth application can be an alternative/additional method of goniometry, easier to use than other photographic-based goniometric assessments. Further studies are required to assess its reliability for the measurement of other joints.