Diamond nanoparticles (DNPs), also known as nanodiamonds, have attracted significant interest in recent years due to a number of potential applications. Their particular usage requires proper surface engineering. In this work, DNPs with a nominal diameter of 5 nm were treated using underwater pulsed streamer corona discharge. A reactor with a needle-to-plate electrode system was employed. The electrolytic conductivity of aqueous DNPs suspensions (0.37 g l⁻¹) was adjusted by NaCl to 100 and 500 μS cm⁻¹. The discharge-treated particles predominantly formed several mm long filaments consisting of agglomerates with submicron diameter, independent of the solution conductivity and the treatment time. The treatment of DNPs decreased the sp²-bonded carbon atoms, as evaluated by XPS for more conductive solution. For both solutions, oxidation of the DNP surface was observed. FTIR measurements showed evolution of new bands at 800–950 cm⁻¹ and 1261 cm⁻¹, which were attributed to the formation of epoxides via the attack of HO₂˙ radicals on surface CC double bonds.