We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene. Pentasilicene has been recently reported, 1 while the stable penta-germanene, belonging to the same group IV element, is revealed firstly by our ab initio calculations. We find that both pentasilicene and penta-germanene at room temperature have ultralow lattice thermal conductivities κ of 1.29 W/mK and 0.30 W/mK respectively . To the best of our knowledge, pentagermanene may have the lowest κ in 2D crystal materials. We attribute ultralow κ to the weak phonon harmonic interaction and strong anharmonic scattering. A small phonon group velocity, a small Debye frequency, a large Grüneisen parameter, and a large number of modes available for phonon-phonon interplay together lead to the ultralow κ of penta-silicene and pentagermanene. These discoveries provide new insight into the manipulation of ultralow κ in 2D materials and highlight the potential applications of designing silicon and germanium based high thermoelectric materials. 2 arXiv:1910.13780v1 [cond-mat.mtrl-sci]