Cl⁻ transport in a conductive polymer (CP) film was demonstrated for n-type thermoelectric (TE) harvesting. CPs have been considered as an important group of p-type TE materials due to their high TE functionalities plus simple processing steps for a device. In particular, recently emerging p-type ionic CPs can be unique candidates due to their high Seebeck coefficients (S). However, n-type materials based on CPs suffer from very poor TE functionalities, and n-type ionic TE CP materials have not been realized so far. Here, we report the first example of n-type mixed ionic–electronic CP composite (NPC) films. The p-type TE properties of the PEDOT:PSS films was drastically converted into the n-type TE properties in the presence of CuCl₂ through metal binding with polymers, thus resulting in the formation of Cl⁻ channels. Fluorescence imaging using Cl⁻ as an indicator and time-of-flight secondary ion mass spectrometry mapping confirmed that Cl⁻ is transported in the film from the hot to the cold electrode. In addition, electron spin resonance spectroscopy indicated the major spin density transition from a polaron of PEDOT:PSS to the polymer-bound unpaired electron spin of Cu ions by increasing the CuCl₂ content to prove the binding of metal ions with the PSS unit of the polymer chain. These mixed ionic–electronic NPC films recorded a surprisingly high negative S value of over −18.2 mV K⁻¹ and a power factor of 1.7 mW m⁻¹ K⁻² at 80% RH with 40 wt% of CuCl₂. Taking advantage of this high performance, the CP films were integrated with a p-type CP film as a flexible module-type TE harvester with 10 pairs of p–n legs on CNT electrodes. This TE harvester showed a thermovoltage of 1.55 V for a low temperature gradient of 4.5 K. This high anion transport in a TE CP hydrogel film might be a useful solution for environmentally benign and body-worn electronics.