We introduce an innovative design of a disk triboelectric nanogenerator (TENG) with segmental structures for harvesting rotational mechanical energy. Based on a cyclic in-plane charge separation between the segments that have distinct triboelectric polarities, the disk TENG generates electricity with unique characteristics, which have been studied by conjunction of experimental results with finite element calculations. The role played by the segmentation number is studied for maximizing output. A distinct relationship between the rotation speed and the electrical output has been thoroughly investigated, which not only shows power enhancement at high speed but also illuminates its potential application as a self-powered angular speed sensor. Owing to the nonintermittent and ultrafast rotation-induced charge transfer, the disk TENG has been demonstrated as an efficient power source for instantaneously or even continuously driving electronic devices and/or charging an energy storage unit. This work presents a novel working mode of TENGs and opens up many potential applications of nanogenerators for harvesting even large-scale energy.