The detection and identification of chemical warfare agents (CWAs) are critical to safeguarding global security, as these toxic substances pose significant threats to human health and the environment. Effective monitoring and control of CWAs are essential for compliance with the Chemical Weapons Convention. Contamination of water sources with CWAs or their degradation products can have long-lasting ecological and public health implications. Dimethyl methylphosphonate (DMMP), a chemical simulant with structural similarities to hazardous organophosphorus agents, was selected for this study due to its relevance in proficiency testing (PT) and method validation. Here, we present the development and validation of a gas chromatography-mass spectrometry technique for the determination of DMMP. The QuEChERS extraction method was utilized to enhance sample preparation efficiency. The results obtained from the validated method revealed excellent linearity (R² = 0.9998), a low limit of detection of 0.0167 ppm, and high accuracy and precision, with recovery values between 95.7% and 97.3%, and low relative standard deviations at 3.5% for intraday and 3.7% for interday. The method was successfully applied in an Organisation for the Prohibition of Chemical Weapons PT, achieving a recovery value of 95.6% for DMMP. These results demonstrate the reliability of the method, underscoring its potential for use in international efforts to monitor and control CWAs, thereby preventing their misuse.