The organic ionic plastic crystal (OIPC) family of electrolyte materials has provided a universal material platform for many electrochemical devices such as all-solid-state lithium ion batteries and medium-temperature (T = 120∼180 °C) fuel cells. These OIPC materials usually benefit from being compounded with various matrix materials which not only provide mechanical support but also dramatically affect the ion transport within the composites by forming a percolated interfacial region and/or changing the solid-state structure and dynamics of the bulk phase. Therefore, a fundamental understanding of the influence of the matrix on the ion dynamics of the OIPCs is crucial. In this work composite membranes based on protic organic ionic plastic crystal (POIPC) 1-(N,N-dimethylammonium)-2-(ammonium)ethane triflate ([DMEDAH2][Tf]2) and poly(vinylidene difluoride) (PVDF) have been prepared and characterized systematically. Particular attention has been paid to the influence of PVDF nanofibers on the thermal properties, phase behavior, ionic conduction, and molecular dynamics of the OIPC ions. We found that the presence of PVDF nanofibers reduces the mobility of the OIPC molecules at the interfacial region, while in the meantime it also dramatically changes the crystalline structure and ion dynamics of the bulk OIPC phase. Solid-state NMR revealed both lower mobile component content and significantly reduced molecular dynamics in the composites. This result is highly consistent with the DSC data which show a notably higher melting enthalpy for the composite. Combination of DSC, solid-state NMR, and FTIR techniques consistently explained the fundamental mechanisms of the decreased ionic conductivity of the OIPC/PVDF composite material. Variable-temperature synchrotron XRD results suggest that thermal history plays an important role in modifying the POIPC crystal structure and symmetry, and the addition of PVDF nanofibers tends to stabilize the metastable phase of the POIPC material. This work highlights the importance of understanding the OIPC-matrix interactions and the resultant interface when designing future solid-state electrolytes.