In this paper, based on the density functional theory, three new two-dimensional pentagonal monolayer semiconductors namely penta-XP5 monolayer (X = Al, Ga, In) are reported. The cohesive energy calculation confirms that the predicted structures are energetically stable. Furthermore, the dynamic and thermal stabilities of the structures are testified by phonon dispersion curves analysis and first principles molecular dynamic simulations respectively. Our calculations show that two-dimensional penta-AlP5, penta-GaP5, and penta-InP5 are indirect semiconductors with tunable wide band gaps (2.69 and 2.71, 2.70 eV respectively). The obtained results suggest these materials as promising candidates for application in new electronic devices in nano scale. (C) 2018 Elsevier B.V. All rights reserved.