The afterglow of an atmospheric pressure plasma has been used for the fast oxidative functionalization of multi-walled carbon nanotubes (MWCNTs). Scanning electron microscopy and Raman spectroscopy demonstrate that the MWCNT morphology is mostly preserved when the MWCNTs are dispersed in a solvent and injected as a spray into the plasma. Contact angle measurements show that this approach enhances the wettability of MWCNTs and reduces their sedimentation in an aqueous dispersion. X-ray photoelectron spectroscopy, IR spectroscopy, and electrokinetic measurements show that oxygen plasma incorporates about 6.6 at.% of oxygen and creates mainly hydroxyl and carboxyl functional groups on the MWCNT surface. The typical effective treatment time is estimated to be in the range of milliseconds. The approach is ideally suited for combination with the industrial gas phase CVD synthesis of MWCNTs.