In this work, an ionic conjugated polymer with propargyl side groups (poly(2-ethynylpyridinum bromide); PEP-P) was synthesized. High-quality thin films with smooth surfaces were prepared from this polymer on silicon substrates and metal electrodes, via a conventional, simple solution-coating and drying process. Synchrotron X-ray scattering analysis showed that the polymer in the nanoscale thin films was amorphous, but was somewhat preferentially oriented in the film plane, rather than randomly oriented. Using synchrotron X-ray reflectivity analysis, the electron densities and interfaces between the polymer film layers and the silicon substrate and metal electrodes were examined in detail. To our knowledge, PEP-P is the first ionic conjugated polymer to be shown to exhibit electrically nonvolatile memory behavior. The polymer in the nanoscale thin films showed excellent write-once-read-many-times (WORM) memory characteristics, without any polarity. The switching-ON voltage was lower than 1.5 V. WORM memory devices based on PEP-P were highly stable, even under ambient air conditions. PEP-P therefore has great potential as a candidate material for the low-cost mass production of high-performance, programmable unipolar WORM memory devices with very low power consumption.