Stretchable electronics with excellent elastic characteristics are attracting extensive interest in the area of wearable devices and epidermal electronics. Here, we demonstrate an ultrathin stretchable triboelectric nanogenerator (s-TENG) with coplanar electrode for harvesting diverse biomechanical energies and acting as a self-powered gesture sensor. The s-TENG employs electrospun polyurethane nanofibers and conductive nanomaterials as the stretchable electrode. With the coplanar electrode configuration, the device can generate electricity from diverse working situations, such as folding/unfolding of the device and contact/separation with other objects. Facilitated by the increased contact area of nanostructure and paired electrodes design, the s-TENG can generate enhanced instantaneous peak power density of 316.4 μW cm⁻² when working in the folding/unfolding situation. When in contact with other objects such as cotton cloth and human skin, peak voltages of 330 V and 286 V are obtained, respectively. Thanks to the ultrathin structure of the device, it can be conformally attached on skin and deforms as the body moves. By adjusting the dimensions of the device, the s-TENG can be used to detect human motion in different body parts, showing its great application prospects in sustainable wearable devices, self-powered electronic skins and smart wireless sensor networks.