Abstract: Oxidized nanocarbons (ONCs) have been regarded as efficient electrocatalysts for H₂O₂ production. However, wet chemical procedures involving large volumes of strong acid and long synthetic time are usually needed to obtain these ONCs. Herein, a plasma activation strategy is developed as a rapid and environmentally benign approach to obtain various ONCs, including oxidized multiwalled carbon nanotubes, single-walled carbon nanotube, graphene, and super P carbon black. After a few minutes of plasma activation, oxygen-containing functional groups and defects can be effectively introduced onto the surface of nanocarbons. Enhanced electrocatalytic activity and selectivity are demonstrated by the plasma-ONCs for H₂O₂ production. Taking oxidized multiwalled carbon nanotubes as an example, high selectivity (up to 95%) and activity (0.75 V at 1 mA cm⁻²) can be achieved in alkaline solution. Moreover, ex situ x-ray photoelectron spectroscopy and in situ Raman measurements reveal that C–O, C=O, edge defect, and sp² basal planar defect are probably the active sites.