This study aimed to develop and evaluate a derivatization method using pentafluoropropionic anhydride (PFPA) and 2,2,3,3-pentafluoro-1-propanol (PFPOH) for the confirmatory analysis of 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) in human urine by gas chromatography–tandem mass spectrometry (GC–MS/MS). A total of 30 urine specimens were analyzed, comprising 20 cannabinoid-positive samples and 10 cannabinoid-negative samples previously screened using the DRI Cannabinoid immunoassay. Samples were subjected to solid-phase extraction (SPE) followed by PFPA/PFPOH derivatization prior to GC–MS/MS analysis. The developed method successfully confirmed the presence of THC-COOH in all positive samples and the absence of THC-COOH in all negative samples. Excellent calibration linearity was achieved over the concentration range of 10–120 ng/mL, with a coefficient of determination (r²) of 0.999. Characteristic fragment ions at m/z 445, 459, 607, and 622 provided reliable identification of THC-COOH with library match probabilities exceeding 95%. The method demonstrated satisfactory chromatographic performance and analytical specificity, supporting its suitability for routine forensic and clinical toxicology applications. These findings highlight the value of GC–MS/MS confirmatory testing for cannabinoid detection in urine and support the use of PFPA/PFPOH derivatization as a reliable approach for THC-COOH analysis.