Exploiting humidity-buffering materials for air humidity control is of vital necessities. Novel porous ultrathin NiO nanosheets as effective humidity-buffering materials were successfully synthesized with a simple, low-cost and high-yield method. It exhibits ultrahigh water vapor adsorption-desorption performance at room temperature, which is superior to both commonly used conventional materials and most reported novel ones. The excellent water vapor adsorption capacity can be attributed to its unique porous ultrathin structures. Numerous mesopores with relatively large diameter and volume can largely decrease water molecules diffusion resistance and increase adsorption space. Moreover, the ultrathin structure can lead to more sufficient contact between water vapor and adsorbents and provide great specific surface area, which can further facilitate water vapor adsorption. The potential of ultrathin nanosheets to be humidity-buffering materials has been explored carefully for the first time and this work also sheds light on the rational design of adsorbents with superior water capacity for efficient humidity control.