Monitoring sulfide ions (S2-) in environmental matrices is critical for environmental safety and pollution control. In this work, a novel dual-mode colorimetric and photothermal (PT) point-of-care testing (POCT) platform was developed for S2- assay. The synthesized platinum nanoparticles (Pt NPs) exhibited exceptional oxidase-like activity, catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to its oxidized form (oxTMB), causing a color change from colorless to blue. Additionally, oxTMB exhibits significant PT effects under near-infrared 808 nm laser irradiation. The presence of S2- induced the aggregation of Pt NPs, markedly inhibiting their oxidase-like activity, thereby establishing an inverse relationship between the concentration of S2- and both the RGB value and temperature change in TMB solution. The limits of detection (LODs) were calculated to be 1.5 μM and 4.4 μM for colorimetric and photothermal analysis using a digital thermometer and smartphone, respectively. The S2- in lake water and fuel residue was analyzed, yielding satisfactory results. This method offers low-cost, straightforward, and swift advantages, underscoring its potential for POCT in environmental monitoring.