Background. Several water-based sports (swimming, surfing and stand up paddle boarding) require adequate thoracic mobility (specifically rotation) in order to perform the appropriate activity requirements. The measurement of thoracic spine rotation is problematic for clinicians due to a lack of convenient and reliable measurement techniques. More recently, smartphones have been used to quantify movement in various joints in the body; however, there appears to be a paucity of research using smartphones to assess thoracic spine movement. Therefore, the aim of this study is to determine the reliability (intra and inter rater) and validity of the iPhone ® app (Compass) when assessing thoracic spine rotation ROM in healthy individuals. Methods . A total of thirty participants were recruited for this study. Thoracic spine rotation ROM was measured using both the current clinical gold standard, a universal goniometer (UG) and the Smart Phone Compass app. Intra-rater and inter-rater reliability was determined with a Intraclass Correlation Coefficient (ICC) and associated 95% confidence intervals (CI). Validation of the Compass app in comparison to the UG was measured using Pearson's correlation coefficient and levels of agreement were identified with Bland-Altman plots and 95% limits of agreement. Results. Both the UG and Compass app measurements both had excellent repro- ducibility for intra-rater (ICC 0.94-0.98) and inter-rater reliability (ICC 0.72-0.89). However, the Compass app measurements had higher intra-rater reliability (ICC = 0:96-0:98; 95% CI [0.93-0.99]; vs. ICCD0:94-0:98; 95% CI [0.88-0.99]) and inter- rater reliability (ICCD0:87-0:89; 95% CI [0.74-0.95] vs. ICCD0:72-0:82; 95% CI [0.21-0.94]). A strong and significant correlation was found between the UG and the Compass app, demonstrating good concurrent validity (r D0:835, p < 0:001). Levels of agreement between the two devices were 24.8° (LoA -9.5°, C15.3°). The UG was found to consistently measure higher values than the compass app (mean difference 2.8°, P < 0:001). Conclusion. This study reveals that the iPhone ® app (Compass) is a reliable tool for measuring thoracic spine rotation which produces greater reproducibility of measurements both within and between raters than a UG. As a significant positive correlation exists between the Compass app and UG, this supports the use of either device in clinical practice as a reliable and valid tool to measure thoracic rotation. Considering the levels of agreement are clinically unacceptable, the devices should not be used interchangeably for initial and follow up measurements.