A convenient photocatalyst‐free method for the synthesis of redox‐active 1,2‐dihydro‐3H‐indazol‐3‐one derivatives from (2‐nitroaryl)methanol and amines was developed. The reaction proceeded efficiently at room temperature by irradiation of UV lightunder CO2 atmosphere (1.0 atm, flow) without any photocatalysts or additives. This mild, operationally simple method shows wide functional tolerance. The