Nanoplastics (NPs) are emerging aquatic contaminants due to their persistence, mobility, and potential to transport co-contaminants. However, efficient removal of NPs remains challenging, particularly under water treatment-relevant conditions. This study examined the removal of polystyrene nanoplastics (PSNPs) from water using electrosorption with activated carbon electrodes, with particular focus on the effects of particle size (400–1100 nm), applied voltage (0.4–2.0 V), pH (3.5, 6, and 10.5), and ultraviolet (UV) aging. Removal efficiency increased from 30 to 40% at 0.4 V to approximately 90% at 2.0 V. However, higher voltages were accompanied by increased energy consumption and reduced charge-normalized removal efficiency, indicating a trade-off between removal capacity and charge utilization efficiency. A voltage-dependent size crossover was observed, where smaller PSNPs were preferentially removed under weak electric fields, whereas larger PSNPs exhibited higher removal at stronger fields. Higher pH and UV aging further enhanced removal, likely due to changes in particle surface charge and chemistry. Water matrix composition strongly influenced removal performance, with seawater resulting in high removal, whereas surface water showed lower efficiency due to background organic matter interference. A synthetic electrolyte control (0.55 M NaCl) produced lower removal than actual seawater despite equivalent ionic strength, suggesting that factors beyond ionic strength contribute to enhanced removal. Limited particle recovery (∼21.6% of adsorbed PSNPs) indicated that further optimization of the desorption process is required before practical implementation. Overall, these findings show that PSNP electrosorption is influenced by electrochemical conditions, particle properties, and water matrix composition, providing insight into the design of electrochemical treatment strategies for NP-contaminated waters.