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Abstract Flexibility has been pursued enthusiastically in the field of porous crystals for enhancing their adsorption and separation performances. However, flexibility has never been observed among porous crystals sustained thoroughly by van der Waals interactions since flexible motions readily lead to the collapse of the porous architecture. Here we report a van der Waals crystal featuring conformational flexibility as well as permanent microporosity. The single-crystal structure and its structural transition in response to the adsorption of water molecules were unambiguously disclosed by means of electron and X-ray crystal structure analyses. The peripheral aromatic rings of the constituent molecule rotated as increasing the ambient humidity, while the connectivity of the pores was maintained throughout the structural transition. The transformative pores allowed the guest water molecules to move exceedingly quickly through the pores with a time constant of 490 μs. We demonstrated that the quick release of water induced by photothermal heating induced a significant upward bending of a film set above the crystalline powder compared to conventional porous materials. This finding contributes to the future crystal engineering based on van der Waals interactions rather than cohesive bonds.