Gait analysis is an important clinical tool in the management of locomotion disability arising from orthopedic and neurological conditions and is an active area of research particularly in rehabilitation and sports medicine. The procedure demands dedicated space, thus limiting its usage to indoor environments. In this paper, we present a novel wearable wireless inertial sensor (i-Sens)-based design for estimation of joint angle in the sagittal plane. The i-Sens hardware comprises of triaxial accelerometer, gyroscope sensors, microcontroller and Bluetooth module. It is portable and can be readily used in indoor and outdoor settings. A novel simplified complementary filter algorithm was incorporated in the design for sensor data fusion. Four normal volunteers were chosen to test the sensor; they wore the sensor at knee level on one side, and assessed its performance while walking in both outdoor and indoor environments. The data was transmitted wirelessly through Bluetooth to the data acquisition system. The gait parameters of the subjects were compared with normative reference data. At the initial contact phase of the gait cycle, mean and standard deviation of knee angle was 3.6° ± 3.5°, while the maximum swing phase knee flexion angle was 67.8° ± 4.5°, which is within the normative data range, suggesting accuracy of the joint angle estimation. Hence, i-Sens can be useful for movement analysis of the lower limb. Further clinical trials are required to validate the sensor at different joints (ankle, knee and hip) under various pathological and environmental conditions. © 2018 IEEE.