RbInSn2S6 and CsInSn2S6 are yellow two-dimensional (2D) semiconductors featuring anionic SnS2-type layers of edge-sharing (In/Sn)S6 octahedra. These structures are directly derived from the parent structure of SnS2 by replacement of Sn4+ atoms with A+ and In3+ atoms. The compounds crystallize, isotypic to the ion-exchange material KInSn2S6. They adopt the triclinic space group R3̅m (no. 166). The compounds have similar indirect optical band gaps of 2.31(5) eV for Rb and 2.47(5) eV Cs. The measured work functions for each material are ∼5.38 eV. The density functional theory-calculated effective mass values exhibit strong anisotropy due to the 2D nature of the crystal structures and in the case of CsInSn2S6 for hole carriers along the a, b, and c crystallographic directions are 0.30 m0, 0.34 m0, and 2.54 m0, respectively, while for electrons are 0.06 m0, 0.07 m0, and 0.47 m0, respectively.