Purpose: Health care professionals use smartphones in the clinic with mobile device applications (apps) to measure data such as joint ROM. The purpose of this study was to examine goniometer apps and to compare their measurements to an electronic goniometer gold standard to identify the most precise apps. Method: 7 different apps were identified for Apple and Android devices. Each of these apps provided measurements concurrently with an electronic goniometer reference at 6 different predetermined angles. Descriptive statistics, including mean, standard deviation (SD) from the gold standard electronic goniometer, standard error of mean (SEM), mean absolute difference (MAD), and 95% confidence intervals were calculated. An intraclass correlation coefficient (ICC) was calculated to observe for reliability. Bland-Altman plots were configured to show directional preference of two measurement techniques. A Pearson correlation coefficient (r) was used to determine validity to describe the strength and direction of the relationship for Goniometer Pro (Apple) or 360 Protractor (Android) and the gold standard. Results: The most accurate Apple app was Goniometer Pro and Android app was 360 Protractor. The ICCs for reliability for both the Apple app and the Android app were 0.99 (95% CI 0.98, 1.00). The Pearson correlation coefficient for validity was significant at r=1.00 (95% CI 0.99,1.00) Conclusion: Both Goniometer Pro on the Apple device and 360 Protractor on the Android device were the most accurate with high reliability and validity.