Context: Range of motion is a component of a physical examination used in the diagnostic and rehabilitative processes. Following ankle injury and/or during research, it is common to measure plantar flexion with a universal goniometer. The ease and availability of digital inclinometers created as applications for smartphones have led to an increase in using this method of range of motion assessment. Smartphone applications have been validated as alternatives to inclinometer measurements in the knee; however, this application has not been validated for plantar flexion in the ankle. Objectives: The purpose of this study was (1) to assess the validity of the Clinometer Smartphone Application™ produced by Plaincode App Development for use in the ankle (ie, plantar flexion) and (2) to assess the validity of the inclinometer procedures used to measure ankle dorsiflexion for measuring ankle plantar flexion. Design: Blinded repeated measures correlational design. Setting: University-based outpatient rehabilitative clinic. Participants: A convenience sample (N = 50) of participants (27 females and 23 males) who reported to the clinic (mean age = 30.48 y). Intervention: Patients were long seated on a plinth, with the knee in terminal extension. Three plantar flexion measurements were taken with a goniometer on each foot by the primary researcher. The primary researcher then conducted 3 blinded measurements with The Clinometer Smartphone Application™ following the same procedure. A second researcher, who was blinded to the goniometer measurements, recorded the inclinometer measurements. After data were collected, a Pearson's correlation was calculated to determine the validity of the clinometer app compared with goniometry. Main Outcome Measure: Degrees of motion for ankle plantar flexion. Results: Measurements produced using the Clinometer Smartphone Application™ were highly correlated for right foot (r = .92, P < .001), left foot (r = .92, P < .001), and combined (r = .92, P < .001) with goniometer measurements using a plastic universal goniometer. Conclusion: The Clinometer Smartphone Application™ is a valid instrument for measuring plantar flexion of the ankle.