Production of hydrogen from photocatalytic water splitting provides an alternative strategy to deal with the energy crisis and environmental pollution problems. Despite great efforts have been made to search for desired photocatalysts, it has not yet developed a single photocatalyst material that can satisfy all the requirements for water splitting. Here, we propose that the Janus structures of group-III chalcogenide monolayers (XMMX') are sufficiently efficient photocatalysts for water splitting by the first-principles calculations. Stability analyses reveal that the Janus XMMX' monolayers exhibit energetic and dynamical stability. Moreover, it is possible to realize the Janus XMMX' monolayers via oxidization of the MX' monolayer by the chalcogen X in appropriate experimental conditions. The electronic properties reveal that all of the Janus XMMX' monolayers are semiconductors. Among them, the Janus SGa2Te, SeGa2Te, SIn2Te, and SeIn2Te monolayers are direct gap semiconductors with the desirable band gaps ranging from 1.54 to 1.91 eV. The photocatalytic properties analyses show that the band edge positions of the Janus XMMX' monolayers are suitable for photocatalytic water splitting. Furthermore, the optical absorption spectra of the Janus SGa2Te, SeGa2Te, SIn2Te, and SeIn2Te monolayers exhibit better visible light absorption than the other monolayers. Additionally, the smaller carrier effective masses ensure the photogenerated carriers migrating quickly to participate in the redox reaction of water splitting. Simultaneously, effective spatial separation of photogenerated carriers is also observed in the Janus SGa2Te, SeGa2Te, SIn2Te, and SeIn2Te monolayers. These findings provide valuable guidance for synthesis efforts of the Janus structures of group-III chalcogenide monolayers and for their potential applications as photocatalysts for water splitting.