Conductive carbon nitride, as a hypothetical carbon material demonstrating high nitrogen doping, high electrical conductivity, and high surface area, has not been fabricated. A major challenge towards its fabrication is that high conductivity requires high temperature synthesis, but the high temperature eliminates nitrogen from carbon. Different from conventional methods, a facile preparation of conductive carbon nitride from novel thermal decomposition of nickel hydrogencyanamide in a confined space is reported. New developed nickel hydrogencyanamide is a unique precursor which provides self‐grown fragments of ⋅NCN⋅ or NCCN and conductive carbon (C‐sp2) catalyst of Ni metal during the decomposition. The final product is a tubular structure of rich mesoporous and microporous few‐layer carbon with extraordinarily high N doping level (≈15 at%) and high extent of sp2 carbon (≈65%) favoring a high conductivity (>2 S cm−1); the ultrahigh contents of nongraphitic nitrogen, redox active pyridinic N (9 at%), and pyrrolic N (5 at%), are stabilized by forming NiN bonds. The conductive carbon nitride harvests a large capacitance of 372 F g−1 with >90% initial capacitance after 10 000 cycles as a supercapacitor electrode, far exceeding the activated carbon electrodes that have <250 F g−1. Conductive carbon nitride, simultaneously gaining high nitrogen doping and high electrical conductivity, is obtained from novel space‐confined reaction. Nickel hydrogencyanamide provides self‐grown fragments of ⋅NCN⋅ or NC−CN and conductive carbon (C‐sp2) catalyst of Ni metal during the decomposition. The final product is a tubular structure of extraordinarily high N doping level (≈15 at%) and high conductivity (>2 S cm−1).