Knee range of motion (ROM) following a knee arthroplasty is an important clinical outcome that directly relates to the patient's physical function. Smartphone technology has led to the creation of applications that can measure ROM. The aim was to determine the concurrent reliability and validity of the photo-based application 'Dr Goniometer' (DrG) compared with a universal goniometer performed by a clinician. A smartphone camera was used to take photographs of the knee in full flexion and full extension, and the images were sent by participants to a study phone. Participants then rated the ease of participation. To assess validity, the patient's knee was measured by a clinician using a goniometer. To examine reliability, four clinicians assessed each image using DrG on four separate occasions spaced 1 week apart. A total of 60 images of knee ROM for 30 unicondylar or total knee arthroplasty were assessed. The goniometer and DrG showed strong correlations for flexion (r=0.94) and extension (r=0.90). DrG showed good intrarater reliability and excellent inter-rater reliability for flexion (intraclass correlation coefficient=0.990 and 0.990) and good reliability for extension (intraclass correlation coefficient=0.897 and 0.899). All participants found the process easy. DrG was proven to be a valid and reliable tool in measuring knee ROM following arthroplasty. Smartphone technology, in conjunction with patient-reported outcomes, offers an accurate and practical way to remotely monitor patients. Benefit may be found in differentiating those who need face-to-face clinical consult to those who do not.