The Ag3PO4/C3N4 composites with improved photocatalytic activity were prepared by a facile in situ deposition of Ag3PO4 particles on the surface of C3N4 sheets and characterized by x-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, UV–vis diffuse reflectance absorption spectra, Fourier transform infrared spectra, and photoluminescence spectra. The photocatalytic degradation of Rhodamine B (RhB) over the Ag3PO4/C3N4 composites was investigated and optimized, indicating that the optimal amount of Ag3PO4 in the composites was 90 wt%. The remarkably improved photocatalytic activity of Ag3PO4/C3N4 composites could be attributed to the effective separation of photogenerated charge carriers. The photoelectrochemical measurements confirmed that the charge separation efficiency was improved for the formation of composites. Moreover, the tests of radical scavengers demonstrated that h+ and ·O2− were the main active species for the degradation of RhB.