A simply designed, highly sensitive, stretchable, compact wearable, and skin-like optical fiber sensing instrument is designed and demonstrated for joint motion monitoring. The fiber sensing scheme comprises only a section of single-mode fiber (SMF) deformed in the knot-like configuration, which performs as a Mach-Zehnder interferometer (MZI) based on a modal coupling mechanism between the core and cladding modes of the deformed SMF section. This proposed optical fiber sensor based on a knot-like configuration is mounted onto wearable woven fabric and then garments on the limbs of a healthy human's body. As the flexion angle of the human limb is varied, the interference fringe coding based on the spectral shift difference of the periodical transmission spectra is perceived. The proposed wearable optical fiber sensor exhibits excellent sensitivities from around -0.431 to -0.614 n m / ∘ realized for elbow and knee joint flexion between a range of motion around 0°-90°. Additionally, this sensor also displays high repeatability and stability and a fast response time of 1.4 ms, combined with a small standard deviation of about 2.585%. The proposed sensor device possesses manufacturing simplicity, high processing accuracy, lightness, and elasticity, as well as certain improvements over other goniometers and optical fiber sensors. These attributes of the proposed sensor prove its applicability for human joint angle monitoring.