Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP2 and SiCP4 monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP4 monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼104 cm-1) and high carrier mobility (∼105 cm2 V-1 s-1) of SiCP2 and SiCP4 facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP2 and SiCP4 monolayers promising candidates for applications in photocatalytic water splitting.