An efficient and eco-friendly injection-port tert-butyldimethylsilylated (TBDMS) derivatization and gas chromatography-mass spectrometry (GC/MS) were developed to determine an antibacterial agent, triclosan (TCS), and its metabolite: methyltriclosan (MTCS), in wastewater and surface water samples. The effects of several parameters related to the TBDMS-derivatization process (i.e., injection-port temperature, residence time and volume of silylating agent) were investigated. This on-line derivatization-coupled large-volume (10 μL) sample introduction provides sensitive, fast and reproducible results for TCS residue analyses. Each water sample was extracted by reversed-phase C18 solid-phase extraction (SPE) cartridge, and then the recovery efficiency was evaluated using various eluting solutions. Limits of quantitation (LOQs) for MTCS and TCS were 3.0 and 1.0 ng/L in 100 mL of water samples, respectively. Intra- and inter-batch precision with their accuracy were also investigated. The precision for these analytes, as indicated by relative standard deviations (RSDs), proved to be less than 7 and 11%, respectively, for intra- and inter-batch. Accuracy, expressed as the mean recovery, was between 80 and 95%. The method was then applied to environmental water samples, showing the occurrence of TCS in both surface water and municipal wastewater treatment plant (MWTP) influent/effluent samples.