Searching for single-atom thin materials in the planar structure, like graphene and borophene, is one of the most attractive themes in two-dimensional materials. Using density functional theory calculations, we have proposed a series of single-layer planar penta-transition metal phosphides and arsenides, i.e., TM2X4 (TM = Ni, Pd and Pt; X = P, As). According to the calculated phonon dispersion relation and elastic constants, as well as ab initio molecular dynamics simulation results, monolayers of planar penta-TM2X4 are dynamically, mechanically and thermally stable. In addition, screened HSE06 hybrid functional calculations including spin-orbit coupling show that these monolayers are direct band gap semiconductors, with band gaps ranging from 0.14 to 0.77 eV. Ultrahigh carrier mobilities up to 10(4)-10(5) cm(2) V-1 s(-1) both for electrons and holes have been confirmed, among the highest in 2D semiconductors. Our results indicate that planar penta-TM2X4 monolayers are interesting narrow-gap semiconductors with ultrahigh carrier mobility as well as excellent optical properties, and may find potential applications in nanoelectronics and photoelectronics.