Ultrathin two-dimensional (2D) porous materials, which can efficiently capture harmful pollutants from aqueous solutions are highly desired yet scarcely reported. Herein, 2D zeolitic imidazolate framework-67 porous nanosheets (ZIF-67-NS) were first applied as a promising adsorbent for arsenic(iii) removal from water. An ultrathin structure was observed for the as-prepared ZIF-67-NS with an average thickness of 1.3 nm. The resulting ZIF-67-NS shows outstanding water stability in alkaline conditions. Owing to the unique dimensionality, ZIF-67-NS exhibited a significantly higher adsorption capacity (516 mg g-1) and faster uptake kinetics (2 h) toward As(iii) than its 3D bulk-type counterpart (ZIF-67-NB). The superior adsorption performance in the ultrathin ZIF-67-NS is attributed to the coordinatively unsaturated Co(ii) atoms, which are open sites dominating adsorption, as evidenced by X-ray photoelectron and Fourier transform infrared spectroscopy. On the other hand, the phase transformation in ZIF-67-NB was observed from the 3D bulk structure to the 2D layered structure during the As(iii) absorption. Lastly, ZIF-67-NS can be efficiently regenerated with few losses in the adsorption capacity after 3 cycles. Overall, this work represents a practical case of As(iii) removal by a 2D crystalline porous material and demonstrates its potential for the development of 2D nanomaterial adsorbents.