Intraoperative range of motion (ROM) assessment can be challenging during total knee arthroplasty (TKA) surgery. As computer assisted surgery is costly and not readily available to many surgeons, we have developed a simple, cost-effective intraoperative device to precisely measure knee flexion and extension. A simple knee goniometer system was constructed consisting of a digital level mounted to a base that rigidly attaches two standard needles. The needles are pushed through the overlying soft tissue of the distal femur. The device is then applied to the proximal tibia, where an angle measurement of the knee is registered. A validation study for this device was conducted on two pairs of intact cadaveric lower limbs at 0 deg, 10 deg, 15 deg, 20 deg, 25 deg, and 30 deg. Two orthopedic surgeons experienced with the system performed three measurements at each angle. Systematic error, defined as the goniometer reading at 0 deg flexion anatomically as determined by the navigation system, ranged from −9.1 deg to 3.0 deg, consistent for each operator on every case. Measurement error, defined as the variability in repeated, fixed angle measurements made with the goniometer, was 1.5 ± 1.0 deg across all surgeons, cases, and prescribed flexion angles. For both surgeons and all imposed flexion angles, measurement errors were below the 4 deg clinical threshold. The simple knee goniometer system generated accurate, repeatable measures of changes in flexion angle intraoperatively with measurement error comparable to errors obtained using the commercial navigation system (1 deg–2 deg). However, the knee goniometer is less complex, less time intensive, and less costly than currently available computer assistive devices. Taken together, our results are very promising for the continued development of this device.