Herein, in situ surfactant-free synthesis of ultrathin BiOCl/g-C3N4 (BOC/CN) nanosheets for enhanced visible-light photodegradation of rhodamine B was described. The formation procedure, microstructural morphology, structure, optical properties, and photocatalytic activity were researched in detail. The results show that ultrathin BOC/CN heterojunctions nanosheets were successfully constructed via one-step surfactant-free solvothermal method by introducing CN into the synthesis system of BOC, and the thickness of BOC/CN nanosheets was similar to 3.5 nm. The ultrathin 2D/2D BOC/CN heterojunctions significantly enhanced the photodegradation of RhB dye under visible-light irradiation. The best-performing sample BOC/CN-60 had 3.9 and 12.8 times higher photocatalytic efficiency for visible-light RhB removal compared to pure BOC and CN, respectively. The large enhancement in the photodegradation activity should be attributed to the synergistic effect of BOC and CN in BOC/CN nanosheets. The synergism reflects in the widened optical window for effective light absorption, enlarged interfacial area for more efficient electron-hole separation, and a shorter diffusion distance for facilitated charge transfer. A possible visible-light RhB photodegradation mechanism was proposed, based on theoretical analysis and active species trapping experiments.