High-performance electrocatalysts for water splitting at all pH values have attracted considerable interest in the field of sustainable hydrogen evolution. Herein, we report an efficient electrocatalyst with a nanocrystalline cobalt phosphide (CoP) network for water splitting in the pH range of 0-14. The novel flexible electrocatalyst is derived from a desirable nanocrystalline CoP network grown on a conductive Hastelloy belt. This kind of self-supported CoP network is directly used as an electrocatalytic cathode for hydrogen evolution. The nanocrystalline network structure results in superior performance with a low onset overpotential of ~45 mV over a broad pH range of 0 to 14 and affords a catalytic current density of 100 mA·cm^-2 even in neutral media. The CoP network exhibits excellent catalytic properties not only at extreme pH values (0 and 14) but also in neutral media (pH = 7), which is comparable to the behavior of state-of-the-art platinum-based metals. The system exhibits an excellent flexible property and maintains remarkable catalytic stability during continuous 100-h-long electrolysis even after 100 cycles of bending/extending from 100° to 250°.