We investigate the hydrogen adsorption on and diffusion through the MoS2 monolayer based on density-functional theory. We show that the hydrogen atom prefers to bond to the S atom at the monolayer, leading to enhanced conductivity. The hydrogen atom can also adsorb at the middle of the hexagon ring by overcoming an energy barrier of 0.57 eV at a strain of 8%. Also, we show that the MoS2 monolayer is flexible and any mechanical deformation of the monolayer is reversible because the extension of the Mo-S bond is much smaller than the applied strain. The monolayer can block the diffusion of hydrogen molecule from one side to the other due to a high energy barrier (6.56 eV). However, the barrier can be reduced to 1.38 eV at a strain of 30% and even totally removed by creating S vacancies and applying a strain of 15%. The MoS2 monolayer may find applications in sensors to detect hydrogen, and as mechanical valve to control the concentration of hydrogen gas. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.