Wearable electronics capable of measuring three-dimensional knee joint angle would provide new methods to predict both anterior cruciate ligament (ACL) injuries and the risk of developing early knee osteoarthritis. However, knee joint angle assessment is currently limited, due to the lack of validated wearable and untethered technologies that can be deployed in natural environments and rural or community settings. To address this challenge, we created a fully untethered, wearable electronic device to continuously measure knee joint angle using two inertial measurement unit (IMU) sensors. The wearable device is composed of a stretchable circuit assembled on a spandex-blend fabric substrate that allows the device to conform to the knee without restricting natural human movement. The fabric substrate allows the electronic circuit to be reused, while increasing the robustness of the device. Finally, we demonstrate the ability of the device to continuously measure sagittal plane knee joint angle during natural human movements outside of a laboratory environment.