Discovery of structure–property interrelations in organic electrochemical transistors (OECTs) is limited by the small number of high-performing semiconducting polymer families that are electrochemically active in aqueous media. Currently, state-of-the-art polymers often come with processability drawbacks; aqueous-processable polymers, such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) require insulating cross-linkers to protect against dissolution in aqueous electrolyte, while glycolated polymers frequently exhibit marginal solubility in both organic and aqueous solvents. Herein, we show that the carboxylic acid-functionalized conjugated polymer poly [3-(4-carboxypropyl)thiophene] (P3CPT) can be processed from a water-soluble precursor, yet requires no additives to yield solvent-resistant OECTs which exhibit electroactivity in aqueous and organic electrolytes. Devices fabricated with P3CPT exhibit unipolar p-channel operation in accumulation mode, with maximum transconductance of 26 ± 2 mS on interdigitated electrodes and competitive volumetric capacitance (C*) of 150 ± 18 F-cm^–3, which rank amongst the highest for conjugated polymers with ionic side chain moieties. This work paves the way for future use of carboxylic acid functionalization to modify existing p- and n-channel backbones to yield highly competitive and processable OECT active materials.