Abstract The conversion of bio‐waste into useful porous carbons constitutes a very attractive approach to contribute to the development of sustainable energy economy, even more as they can be used in energy storage devices. Here we report the synthesis of N‐doped carbons from hydrothermal carbonization of macroalgae, Enteromorpha prolifera (EP) , followed by a mild KOH activation step. The obtained N‐doped carbons exhibited surface areas of up to ∼2000 m 2 /g with N‐loadings varied in the range of 1.4∼2.9 at %. By modifying activation temperature, we were able to tune the surface chemistry and porosity, achieving excellent control of their properties. The specific capacitance reached values of up to 200 F/g at 1 A/g in 6 M KOH for the sample obtained at activation temperature of 700 °C (AHC‐ 700 ). The symmetric supercapacitor using the sample activated at 800 °C (AHC‐ 800 ) as electrodes exhibited the highest cycling stability of the samples studied in this work, with capacitance retention of up to 96 % at 10 A/g, even after 10,000 cycles, constituting the highest reported for biomass‐derived carbon electrodes. These results show the great potential of N‐doped carbons as electrodes for supercapacitors and confirm the excellent electrochemical properties of biomass‐derived carbons in energy storage technologies.