Context: Technological advances have given smartphones the capabilities of sensitive clinical measurement equipment at lesser cost and higher availability. The Clinometer is a smartphone application that can be used to measure the joint range of motion in a clinical setting, but psychometric properties of the tool’s use measuring cervical range of motion (CROM) are not established. Objectives: The purpose of this study was to examine the validity and intrarater reliability of the Clinometer application for the measurement of CROM (ie, flexion, extension, rotation, lateral flexion) and to determine the minimal detectable change and SEM. Design: A blinded, repeated-measures correlational design was employed. Setting: The study was conducted collaboratively between 2 athletic training clinics. Participants: A convenience sample of healthy adults ages 18–30 years were recruited. Participants with any history in the last 3 months of cervical or thoracic pathology, pain, or any musculoskeletal injury were excluded. Main Outcome Measures: Three repetitions of each motion were measured by a primary researcher with a goniometer. The same researcher then conducted 3 blinded measurements with the Clinometer application following the same procedure. A second researcher, blinded to the goniometer measurements, recorded the results. Thirty minutes later, testing was repeated with the application. The Pearson correlation was calculated to determine validity of the application compared with goniometry. Results: The measurements between devices had moderate to excellent concurrent validity, with the coefficients ranging between 0.544 and 0.888, P < .01. Test–retest reliability of the CROM measurement using the application was moderate to excellent, with intraclass correlation coefficients ranging between .774 and .928. Across all movements, the SEM ranged from 1.17° to 2.01°, and the minimal detectable change ranged from 1.18° to 2.02°. Conclusion: The Clinometer application is a valid and reliable instrument for measuring active CROM. Level of evidence: clinical measurement, level 1b.