Nitrogen functionalization (ca. 4 at. % NXPS) of a highly microporous activated carbon (SBET > 3000 m2/g) has been achieved by two different approaches at mild conditions: (i) oxidation and post-reaction with nitrogen reactants, and (ii) direct reaction of pristine carbon material with nitrogen reactants. Interestingly, the introduction of nitrogen functionalities allows full preservation of the microporosity when pathway (ii) is followed. The electrochemical performance of the carbon materials as electrodes for supercapacitors was evaluated by using symmetric and asymmetric configuration in 1 M H2SO4. Both nitrogen-functionalized carbon materials showed larger stability and energy efficiency than the pristine carbon material when working at 1.4 V. The non-oxidized and functionalized activated carbon evidences the best performance as electrode for supercapacitor, providing energy and power density of 14.5 Wh/kg and 61.2 kW/kg and keeping 83% of original capacitance after 5000 charge–discharge cycles. This improvement is related to the presence of surface nitrogen functionalities that provide a higher electrochemical stability, avoiding the formation of detrimental oxygen groups during the operation of the supercapacitor. ; This work was supported by the Ministry of Economy and competitiveness of Spain (MINECO) (CTQ2015-66080-R, MAT2013-42007-P) and Generalitat Valenciana (PROMETEO/2013/038 and PROMETEOII/2014/010). RRR thanks MINECO for ‘Juan de la Cierva’ contract (JCI-2012-12664). MJML acknowledges Generalitat Valenciana for the financial support through a VALiþd contract (ACIF/2015/374).