Background: With the increasing elderly population, the need for gait analysis of these elderly individuals is also increasing. Most devices are costly and not portable; however, smartphones using built-in sensors capable of measuring motion and are easily available. Objectives: To examine the reliability and validity of knee joint angles of the elderly using smartphone measurements during walking. Design: Quasi-experimental research. Methods: Sixteen elderly people, aged 65+ and living in Daejeon and Chungbuk, South Korea, participated in the study. Electrogoniometers and smartphones were attached to the thigh and the side and front of the shank of each subject, respectively, using double-sided tape, an arm band, and an elastic band. Each subject completed two sets of at least seven gait cycles (14 steps). Results: Both the smartphones and electrogoniometers exhibited high agreement in terms of their primary and secondary measurements (ICC>.75). The agreement between the smartphones and electrogoniometers was also high in terms of both the primary and secondary measurements (ICC<.60). Conclusion: These results indicate that smartphones can be costly equipment cannot, even though they cannot completely replace existing clinical-grade devices. Their utility is emphasized herein for measuring knee joint angles of the elderly during walking.