The determination of selenium (Se) has garnered increasing interest because of its importance for life as a nutrient element as well as its toxicology. Herein, a simple multimodal colorimetric method was developed for the detection of selenium. Firstly, thiol cotton was selected as the functional material for simple solid-phase extraction separation of Se(IV). Then, the enriched Se(IV) oxidizes I- ions into I2 species, leading to the formation of a linear triiodide ion that intercalates into the coil of starch molecules, producing a blue color. Based on the changes in the intensity of the blue color, multimodal detection methods using spectrophotometric detection, smartphone app-based colorimetric detection, and naked-eye colorimetric detection were developed for selenium detection. The spectrophotometric detection mode exhibited a linear response from 0.01 to 8.0 μg/mL with a detection limit of 0.0047 μg/mL; the smartphone app-based colorimetric detection mode was linear from 1.0 to 6.0 μg/mL with a detection limit of 0.05 μg/mL; and seleniumlevels of 0.1 μg/mL could be clearly distinguished by the naked eye. The colorimetric detection method was demonstrated to be applicable for the detection of selenium in real water and vegetable samples, giving recoveries of 94.1 to 108.4%.