Abstract Preserving large permanent pore structures in a fluid may endow conventional liquids with emergent physical properties. However, such materials are challenging to make because of the tendency of the pores to be filled and occupied by the solvent molecules. Here, we report the design and synthesis of the first Type III porous liquid (PL) containing uniform yet stable 480 nm cavities. This was achieved by first constructing a single crystalline hollow metal–organic framework (MOF), UiO‐66‐NH2, through chemical etching. The thin yet defect‐free MOF shell effectively excludes the bulky poly(dimethylsiloxane) solvent molecules from entering the cavity through its 4 Å aperture, resulting in the preservation of both micro‐ and macroporosity in the PL. These enormous void spaces allow the PL to reversibly host and release up to 27 wt % water for up to 10 cycles. The switching between the “dry” state and the “wet” state led to a large changes of the thermal conductivity of the PL from 0.140 to 0.256 W m−1 K−1, affording a guest‐responsive liquid thermal switch with a switching ratio of 1.8.
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