Water disinfection using processes such as chlorination is required to kill bacteria and harmful biological components. During chlorination, organic components in water react with chlorine, forming harmful disinfection by-products such as trihalomethanes (THMs). These compounds are very harmful to humans, animals, and plants. Thus, the concentration of these substances in groundwater as well as the seasonal variation in this concentration are of immense interest to scientists. A headspace method was used to analyze trihalomethanes (THMs) using an Agilent 6890N GC/MS. Five-milliliter water samples were employed in this analysis. Separation was performed on a J&W-VRX column. A selected ion monitoring (SIM) mode that complied with US EPA method no. 501 was developed for the detection of four THM species. Quantification and method validation were performed using external standard calibration. The total trihalomethane formation potentials (TTHMFPs) in four groundwater wells in the summer and the winter were found to be below the maximum limits specified by environmental agencies. Seasonal variations were more pronounced for the formation of chloroform (CF), which was the dominant THM species formed. The fraction of the TTHMFP that was due to CF increased in the winter in groundwater samples from the Anabta (2), Anabta (3), and Al Rehya wells, whereas the fraction of the TTHMFP that was due to CF was almost the same in summer (43.2%) and in winter (40.3%) in the groundwater samples from the Al Fawar well. These findings are important for the appropriate regulation of THM levels and for achieving a better understanding of environmental public health and epidemiological issues concerning disinfection by-products in Palestine.