Nanofluidics　derived　from　low-dimensional　nanosheets　and　protein　nanochannels　are　crucial　for　advanced　catalysis,　sensing,　and　separation.　However,　polymer　nanofluidics　is　halted　by　complicated　preparation　and　miniaturized　sizes.　This　work　reports　the　bottom-up　synthesis　of　modular　nanofluidics　by　confined　growth　of　ultrathin　metal-organic　frameworks　(MOFs)　in　a　polymer　membrane　consisting　of　zwitterionic　dopamine　nanoparticles　(ZNPs).　The　confined　growth　of　the　MOFs　on　the　ZNPs　reduces　the　chain　entanglement　between　the　ZNPs,　leading　to　stiff　interfacial　channels　enhancing　the　nanofluidic　transport　of　water　molecules　through　the　membrane.　As　such,　the　water　permeability　and　solute　selectivity　of　MOF@ZNPM　are　one　magnitude　improved,　leading　to　a　record-high　performance　among　all　polymer　nanofiltration　membranes.　Both　the　experimental　work　and　the　molecular　dynamics　simulations　confirm　that　the　water　transport　is　shifted　from　high-friction-resistance　conventional　viscous　flow　to　ultrafast　nanofluidic　flow　as　a　result　of　rigid　and　continuous　nanochannels　in　MOF@ZNPM.