ABSTRACT Objectives: The aim of this study was to assess the concurrent validity and reliability of the Simple Goniometer (SG) iPhone® app compared to the Universal Goniometer (UG). Design: Within subject comparison design comparing the UG with the SG app. Setting: James Cook University, Townsville, Queensland, Australia. Participants: Thirty-six volunteer participants, with a mean age of 60.6 years (SD 6.2). Interventions: Not applicable. Main outcome measure: Thirty-six participants performed three standing lunges during which the knee joint angle was measured with the SG app and the UG. Results: There were no significant differences in the measures of individual knee joint angles between the UG and the SG app. Pearson correlations of 0.96-0.98 and intraclass correlation coefficients of 0.97-0.99 (95% confidence interval: 0.95-1.00) were recorded for all measures. Using the Bland-Altman method, the standard error of the mean of the differences and the standard deviation of the mean of the differences were low. Conclusion: The measurements from the SG iPhone® app were reliable and possessed concurrent validity for this sample and protocol when compared to the UG.