Background: The most widely used tool for assessment of range of motion is the Universal Goniometer. An accurate Goniometer reading depends on the accurate placement of the fulcrum over the centre of rotation of the joint and the correct alignment of the arms to proximal and distal bony landmarks. Differences in educational and experiential background have been implicated as measurement factors in goniometric accuracy. Since smartphones are becoming more accessible and portable, clinicians are exploring their use as a clinical tool including several applications specifically designed for ROM measurements. Method: 100 young adults in the age group of 19-35 were evaluated for range of motion of knee flexion by the tester using a Universal goniometer followed by the smartphone application and the readings were recorded. After an hour the same procedure was followed by the inter rater with both instruments. Results: Reliability and Validity of the smartphone application and universal goniometer was analyzed using ICC and Pearson’s correlation. For the UG, ICC at 95% CI for intra rater reliability is 0.997 and for inter rater reliability is 0.991. For the smartphone application, ICC at 95% CI for intra rater reliability is 0.999 and for inter rater reliability is 0.999. Intra rater reliability of UG vs smartphone application with ICC at 95% CI is 0.997. There was a strong positive correlation to confirm concurrent validity between the smartphone goniometer application and the Universal Goniometer (r=0.9931; p<0.0001) in this study. Conclusion: The results of the study suggest that the smartphone app has a good intra and inter-rater reliability and concurrent validity with the UG.