<p>Viologen derivatives are widely used redox-active motifs whose reversible redox chemistry between the dication (V²⁺) and the radical cation (V^•+) is often accompanied by the formation of a π-bonded dimer. While such aggregation has been extensively studied in molecular systems and polymer solutions, the influence of spatial confinement on the redox behavior of viologen-functionalized polymers remains poorly understood. Here, we report the synthesis of a viologen-functionalized polymer, poly(4-vinylpyridine-<i>N</i>-diethylviologen) (P4VP-V), prepared via atom-transfer radical polymerization followed by postpolymerization functionalization and investigate its electrochemical behavior in aqueous solution and as a surface-grafted polymer brush. In solution, electrochemical reduction of viologen units increases the hydrophobicity of the polymer chains, leading to reversible coil–globule transition and aggregation. Cyclic voltammetry reveals that the apparent number of electrons transferred increases at slower scan rates, which is consistent with cooperative electron transfer associated with viologen radical dimerization. In contrast, when P4VP-V is confined within a polymer brush on an indium tin oxide (ITO) substrate, the viologen units undergo only a single-electron redox process. These observations indicate that spatial confinement and chain crowding within the polymer brush limit the association of the viologen units required for radical dimerization, demonstrating that nanoscale confinement fundamentally alters intermolecular interactions and electron-transfer behavior.</p>