Introduction of functional groups on the hypercrosslinked microporous polymer can significantly enhance its performances and applications, but direct synthesis of the hypercrosslinked microporous functional polymer by an “external crosslinking” technique remains highly challenging. Herein, we reported initial synthesis of hypercrosslinked microporous functional polystyrene (HCPMOS) originated from poly(para-methoxystyrene) (PMOS) by a Friedel-Crafts alkylation reaction using formaldehyde dimethyl acetal (FDA) as an external crosslinker. The hypercrosslinked structure of HCPMOS was fully characterized, and HCPMOS had a microporous structure (0.64–1.37 nm) and high surface area (813 m²/g). The hypercrosslinked microporous poly(para-methoxystyrene) (HCPMOS) showed selective and good adsorption capability for Fe³⁺ removal from aqueous solution. FTIR and X-ray photoelectron spectroscopy (XPS) proved that enhanced adsorption capability of HCPMOS toward Fe³⁺ was contributed by both chelating capability of methoxy groups toward metal and large surface areas. Adsorption performances of HCPMOS were investigated at different Fe³⁺ concentrations, contact times, and temperatures. Adsorption equilibrium and kinetic process of Fe³⁺ on HCPMOS were well described by the Langmuir isotherm model and the pseudo-second-order kinetic model. HCPMOS showed great potential as a selective and good adsorbent for removal of Fe³⁺ from polluted water.