For the first time to our knowledge, graphitic carbon nitride (g-C3N4) nanosheets are found to be an excellent saturable absorber material in the visible waveband. g-C3N4 exhibits much stronger saturable absorption in this region than in the near-infrared region, unlike other two-dimensional materials such as graphene and black phosphorus. By the Z-scan method, the nonlinear absorption coefficient β of the material is first measured at three visible wavelengths, and for g-C3N4 it is 2.05, 0.34, and 0.11 cm·GW 1 at 355, 532, and 650 nm, respectively. These are much larger than 0.06 cm·GW 1 at 1064 nm.