Capacitive deionization (CDI) is energetically favorable for desalinating low-salinity water. The bottlenecks of current carbon-based CDI materials are their limited desalination capacities and time-consuming cycles, caused by insufficient ion-accessible surfaces and retarded electron/ion transport. Here, we demonstrate porous carbon fibers (PCFs) derived from microphase-separated poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as an effective CDI material. PCF has abundant and uniform mesopores that are interconnected with micropores. This hierarchical porous structure renders PCF a large ion-accessible surface area and a high desalination capacity. In addition, the continuous carbon fibers and interconnected porous network enable fast electron/ion transport, and hence a high desalination rate. PCF shows desalination capacity of 30 mgNaCl g⁻¹ PCF and maximal time-average desalination rate of 38.0 mgNaCl g⁻¹ PCF min⁻¹, which are about 3 and 40 times, respectively, those of typical porous carbons. Our work underlines the promise of block copolymer–based PCF for mutually high-capacity and high-rate CDI.