Abstract: Two-dimensional nanochannel membranes offer great opportunities for developing efficient and robust devices for osmotic energy harvesting. However, low counterion concentration associated with the low charge density of nanosheets restricts their output performance. Herein, graphene oxide (GO) and poly(sodium 4-styrenesulfonate) (PSS) were assembled into composite nanochannel membranes featuring two-dimensional (2D) channels intercalated with abundant surface charges. The effect of ionic strength, salt concentration gradient, PSS content, and polyelectrolyte type on the transmembrane ionic transportation and osmotic energy harvesting of GO-PSS composite membranes was investigated. In contrast to pristine GO membranes, the incorporation of PSS simultaneously improves the ionic permeability and ion selectivity of GO-PSS composite membranes, thus leading to its higher output power density than that of pristine GO membranes. The GO-PSS composite membranes offer an output power density up to 11.27 W·m⁻² by mixing seawater and river water, much higher than 3.37 W·m⁻² of conventional GO membranes. This work highlights the significance of charge density and presents a general strategy for effectively improving ion transport through two-dimensional nanochannel membranes for high-output osmotic energy harvesting.