Keggin-type polyoxometalate (POM)-based compounds have long been considered as one of the environmentally friendly candidates of solid electrolytes exhibiting high proton conductivity under high relative humidity (RH >95%). However, their application has been limited by the lack of structural stability and large decrease in conductivity with a slight decrease in RH. In order to overcome these disadvantages, we report a series of crystalline composites based on Preyssler-type POMs ([Na(H2O)P5W30O110](14-), [Bi(H2O)P5W30O110](12-)), which are known to show higher acidity in comparison to Keggin-type POMs, and polymers (polyethylene glycol (PEG), polyallylamine (PAA)). Electrostatic interactions between POMs and polymers contribute to enhance the structural stability, and it has been widely known that polymer electrolytes promote cation transport via segmental motion of the polymer chain. In the crystalline composites, K+ acts as a linker to connect the POMs three-dimensionally, resulting in an all-inorganic framework, and polymers and waters of crystallization reside in the framework. The composites with PEG exhibit moderate proton conductivities of 10(4) S cm(-1) under nonhumidified (RH <10%) and low-temperature (<368 K) conditions by the aid of segmental motion of PEG. The composite with PAA exhibits a high proton conductivity of 10(3) S cm(-1) under humidified (RH 75%) and low-temperature (<338 K) conditions.