Mesoporous ferrihydrite/SiO2 composites were synthesized according to a water-in-oil microemulsion method and characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry, nitrogen-adsorption/desorption, and by X-ray photoelectron spectroscopy. The as-prepared porous ferrihydrite/SiO2 composites showed an excellent adsorption performance for formaldehyde (HCHO) removal from indoor air at ambient temperature. It was found that the aging time during the synthesis had a significant impact on the pore structure, surface area, and HCHO adsorption of these materials. The ferrihydrite/SiO2 composite that was aged for 3 h in the presence of tetraethyl orthosilicate (TEOS) exhibited a relatively high HCHO adsorption capacity, as well as good recyclability, which was attributed to a relatively large BET surface area, optimal pore size, a suitable Si/Fe atomic ratio, and a synergistic effect between ferrihydrite and SiO2. This work not only demonstrates that porous ferrihydrite/SiO2 composites can act as an efficient adsorbent toward HCHO, but suggests a new route for the rational design of cost-effective and environmentally benign adsorbents with high performance for indoor air purification.