Background. Limitation in knee range of motion (ROM) is frequently compromised in patients with patellofemoral pain syndrome (PFPS). Smartphones have become the most widely used device and begun to enter the field of musculoskeletal evaluation. Objective. to investigate the concurrent validity and reliability of the smartphone “Goniometer Records” application in assessing knee ROM in patients with PFPS. Methods. thirty-six PFPS patients (age: 22.82 ± 1.76 years, male:28, female:8) were participated. Knee ROM was measured simultaneously by the smartphone “Goniometer Records” application and by the bubble inclinometer. These measurements were repeated by the same experienced examiner on the same day to measure intra-rater reliability, and by another experienced examiner on the same day to measure inter-rater reliability and after one-week to measure test-retest reliability Results. There was a strong positive correlation to confirm concurrent validity between the smartphone application and the bubble inclinometer (r = 0.97-0.98; p = 0.0001. Intra-rater, inter-rater and test-retest reliabilities of smartphone application suggested excellent reliability for knee flexion and extension (ICC = 0.988-0.997), (ICC = 0.985-0.994), (ICC = 0.984-0.994) respectively. Conclusion. Smartphone application is a valid and reliable alternative for bubble inclinometer in measuring active knee ROM in patients with PFPS.