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A thiourea-based colorimetric sensor incorporating polyethyleneimine (PEI) and chromophoric nitrophenyl groups was synthesized and utilized for detecting various anions. Structural characterization of the sensor was accomplished using FTIR and 1H-NMR spectroscopy. The sensor’s interactions and colorimetric recognition capabilities with different anions, including CI⁻, Br⁻, I⁻, F⁻, NO₃⁻, PF₆⁻, AcO⁻, H₂PO₄⁻, PO₄³⁻, and SO₄²⁻, were investigated via visual observation and UV/vis spectroscopy. Upon adding SO₄²⁻, F⁻, and AcO⁻ anions, the sensor exhibited distinct color changes from colorless to yellow and yellowish, while other anions did not induce significant color alterations. UV/vis spectroscopic titration experiments conducted in a DMSO/H₂O solution (9:1 volume ratio) demonstrated the sensor’s selectivity toward SO₄²⁻, F⁻, and AcO⁻. The data revealed that the formation of the main compounds and anion complexes was mediated by hydrogen bonding, leading to signal changes in the nitrophenyl thiourea-modified PEI spectrum.