Energy-related functionality and performance of organic-inorganic hybrid perovskites, such as methylammonium lead iodide (MAPbI 3 ), highly depend on their thermal transport behavior. Using equilibrium molecular dynamics simulations, it is discovered that the thermal conductivities of MAPbI 3 under different phases (cubic, tetragonal, and orthorhombic) are less than 1 W m −1 K −1 , and as low as 0.31 W m −1 K −1 at room temperature. Such ultralow thermal conductivity can be attributed to the small phonon group velocities due to their low elastic stiffness, in addition to their short phonon lifetimes (<100 ps) and mean-free-paths (<10 nm) due to the enhanced phonon-phonon scattering from highly-overlapped phonon branches. The anisotropy in thermal conductivity at lower temperatures is found to associate with preferential orientations of organic CH 3 NH 3 + cations. Among all atomistic interactions, electrostatic interactions dominate thermal conductivities in ionic MAPbI 3 crystals. Furthermore, thermal conductivities of general hybrid perovskites MABX 3 (B = Pb, Sn; X = I, Br) have been qualitatively estimated and found that Sn-or Br-based perovskites possess higher thermal conductivities than Pb-or I-based ones due to their much higher elastic stiffness. This study inspires optimal selections and rational designs of ionic components for hybrid perovskites with desired thermal conductivity for thermally-stable photovoltaic or highly-effi cient thermoelectric energy harvesting/ conversion applications.