The emergence of wearable electronic products represents the transformation of traditional personal electronic products. Nonetheless, the progress of wearable electronics is greatly limited by the frequent and inconvenient charging of their traditional power units. Therefore, it is desirable to develop a wearable power source that can scavenge biomechanical energy from human motions and then directly power wearable electronics continuously. Here, we demonstrated a naturally wearable cotton triboelectric nanogenerator (C-TENG) based on a piece of cotton in situ on clothes. This wearable C-TENG can power wearable electronics continuously by effectively converting biomechanical energy into electricity. The C-TENG is operated by the friction between the cotton and a porous polytetrafluoroethylene thin film. By naturally integrated on the clothes, the flexible C-TENG can scavenge mechanical energy from the motion of swing arms when people are walking or running. When operated by swinging arms, the output voltage and current of the C-TENG could reach 660 V and 12.5 mu A, respectively. This generated electricity can immediately power some wearable electronics like an electronic watch, a running light and a light-emitting diode guiding sign. This naturally wearable C-TENG could pave the way for developing self-powered personal electronics.