Basil seed-derived multi-heteroatom-doped porous carbons (BHPCs) are successfully synthesized by a facile gelation, followed by a moderate gel water/KOH coactivation process. The BHPC-700 prepared at a relatively low KOH loading and activation temperature possesses large specific surface area (1178.3 m(2) g(-1)), well-defined hierarchical micro/meso porosity, and rich self-doping heteroatom functionalities (13.08 at% of oxygen, nitrogen, phosphorous, and sulfur). Electrochemical tests demonstrate that the BHPC-700-based electrode achieves an ultrahigh specific capacitance (464 F g(-1) at 0.5 A g(-1)), outstanding rate performance (retaining 73.3% capacitance at 50 A g(-1)), and superior cyclic stability (96.8% capacitance retention over 5000 cycles). Furthermore, the BHPC-700 electrodes are assembled into all-solid-state symmetrical supercapacitors. The as-assembled device gives a high energy density of 15.0 Wh kg(-1) at a power density of 500 W kg(-1) and remarkable flexibility, demonstrating great application prospects in the area of sustainable portable electronics.