Searching for promising visible-light photocatalysts for overall water splitting into hydrogen and oxygen is a very challenging task to solve the energy crisis and environment pollution. The widely-used tantalate and niobate perovskite photocatalysts have two drawbacks, i.e., the large energy band gap (similar to 3.2-4.6 eV) and low electron (hole) mobility 10(2) (10(1)) cm(2) V-1 s(-1), which greatly limit their photocatalytic performance. Here, based on the powerful first-principles and accurate GW calculations, we design several novel two-dimensional (2D) Ruddlesden-Popper (RP) type (n = 1) perovskite oxynitrides A(2)BO(3)N (A = Ca, Sr, Ba and B = Ta, Nb) and their bonded heterostructures and comprehensively investigate their interlayer coupling, electronic structures, transport and photocatalytic characteristics. We find that 2D A(2)BO(3)N oxynitrides have a reduced direct band gap at F-point, especially for three-layer (3L) Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure with the optimized band gap similar to 2.0 eV. Compared with tantalate and niobate perovskite oxides, the electron (hole) mobility increases 1-2 orders of magnitude up to 10(3)-10(4) (10(2)-10(3)) cm(2) s(-1). A fast electron-hole vertical transport across the heterointerface and remarkable electron-hole separation can be realized in 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure due to its strong interface Ba-O and Sr-O bonds and type-II band offset. Compared with the well-known photocatalysts, such as BiVO4 and MoS2/g-C3N4, an improved optical absorption (8 x 10(4) cm(-1)) in A(2)BO(3)N is obtained in the visible region. The 2D RP-type perovskite oxynitrides 3L-Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N are powerful visible-light photocatalysts for overall water splitting. (C) 2019 Elsevier Inc. All rights reserved.