N-doped highly microporous carbons have been successfully fabricated from N-rich microalgae by the combination of low-cost hydrothermal carbonization and industry-adopted KOH activation processes. The hydrothermal carbonization process was found to be an essential step for the successful conversion of microalgae into a carbon material. The materials thus synthesized showed BET surface areas in the range ∼1800–2200 m2 g−1 exclusively ascribed to micropores. The carbons showed N contents in the 0.7–2.7 wt.%, owing to the use of N-rich microalgae as a carbon precursor. When tested in symmetric double layer capacitors (occasionally called supercapacitors) based on aqueous LiCl electrolytes, pseudocapacitance was only observable for the sample synthesized at the lowest temperature, 650 °C, which is the one exhibiting the largest amount of N- and O-containing groups. The samples synthesized at 700–750 °C exhibited excellent rate capability (only 20% of capacitance loose at 20 A g−1), with specific capacitances of 170–200 F g−1 at 0.1 A g−1. These materials showed excellent long-term cycling stability under high current densities.