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, NO, PF, AcO, HPO, PO, and SO, were investigated via visual observation and UV/vis spectroscopy. Upon adding SO, 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/HO solution (9:1 volume ratio) demonstrated the sensor's selectivity toward SO, 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.