Strategic installation of heteroatoms and modulation of porous hierarchy of polymers are significant approaches to obtain high-performance materials for practical applications. This study focuses on the syntheses of selenium-containing porous phenolic resin (Se-PR) using cheap raw materials, phenol and selenium dioxide. Two morphologically distinct polymers, Se-PR 1 and Se-PR 2, were obtained via one-step synthesis using DMF and DMSO, respectively, as solvents. Various material characterization techniques such as thermogravimetry, powder X-ray diffraction, BET analysis, X-ray photoelectron spectroscopy, FT-IR spectroscopy and FE-SEM were used to establish the structure and morphology of the polymers. These studies confirmed that the morphology of the polymers is significantly altered by the solvents. Their in-built porous structure with appropriately placed selenium centers in the form of arylselanyl motifs, allows the polymers to display high binding affinity and fast adsorption kinetics towards Hg²⁺ ions. The maximum Hg²⁺ ion uptake capacities for Se-PR 1 and 2 were 625 mg L⁻¹ and 1057 mg L⁻¹, respectively, which highlight the potential of organoselenium polymers as efficient adsorbents for Hg²⁺ removal from water via soft–soft Lewis acid–base interaction (Hg²⁺⋯Se) and underscore their promising role in environmental remediation.