In clinical and research settings, objective range of motion measurement is an essential component of lower limb assessment and treatment evaluation. One reliable tool is the digital inclinometer; however, availability and cost preclude its widespread use. Smartphone apps are now widely available, allowing smartphones to be used as an inclinometer. Reliability and validity studies of new technologies are scarce. Intrarater and interrater reliability of the iHandy Level app installed on a smartphone and an inclinometer were assessed in 20 participants for ankle dorsiflexion using a weight-bearing lunge test. Criterion validity was assessed between a Fastrak and the app, and construct validity was assessed between the inclinometer and the app. Intraclass correlation coefficients2,1 demonstrated excellent intrarater and interrater reliability (intraclass correlation coefficient, 0.97 and 0.76, respectively). Tests of validity demonstrated excellent correlation between all three methods (r > 0.99). The smartphone app is both reliable and valid, provides a low-cost method of measuring range of motion, and can be easily incorporated into clinical practice.