Here, we introduce a highly porous functional mesoscopically silica‐polymer composite based on silica monolith‐conjugated thiourea/formaldehyde copolymer. The developed nanostructure enables selective and fast removal of the radioactive pollutants strontium (Sr[II]) and cesium (Cs[I]) ions from contaminated seawater. The silica/polymer composite was synthesized by introducing thiourea/formaldehyde solution into tetramethoxy orthosilicate/triblock copolymer emulsion. The chemical and textural features of the synthesized silica/thiourea‐formaldehyde polymer composite (SiO2‐TUF) were characterized using Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, scanning electron microscope, high‐resolution transmission electron microscope, energy dispersive X‐ray analysis, dynamic light scattering, thermal analysis, and N2 adsorption/desorption measurements. The synthesized microporous SiO2‐TUF showed excellent cesium and strontium ions removal ability, reaching a maximum adsorption capacity of 78.2 and 40.3 mg g−1 for Sr(II) and Cs(I), respectively. When tested with seawater contaminated with radioactive cesium and strontium, SiO2‐TUF was able to selectively target Sr(II) and Cs(I) ions. Among the different types of adsorption isotherms investigated, Sips isotherm showed the best fit with R2 > 0.990. The kinetic studies showed that the pseudo‐second‐order model gave the best description of the uptake process.