AbstractHumic acids, a major constituent of natural organic carbon resources, are naturally formed through the microbial biodegradation of animal and plant residues. Due to numerous physiologically active groups (phenol, carboxyl, and quinone), the biomedical applications of humic acid have been already investigated across different cultures for several centuries or even longer. In this work, sodium humate, the sodium salt of humic acid, is explored as phototheranostic agent for light‐induced photoacoustic imaging and photothermal therapy based on intrinsic absorption in the near‐infrared region. The purified colloidal sodium humate exhibits a high photothermal conversion efficiency up to 76.3%, much higher than that of the majority of state‐of‐the‐art photothermal agents including gold nanorods, Cu9S5 nanoparticles, antimonene quantum dots, and black phosphorus quantum dots, leading to obvious photoacoustic enhancement in vitro and in vivo. Besides, highly effective photothermal ablation of HeLa tumor is achieved through intratumoral injection. Impressively, sodium humate reveals ultralow toxicity at the cellular and animal levels. This work promises the great potential of humic acids as light‐mediated theranostic agents, thus expanding the application scope of traditional humic acids in biomedical field.