A simple, fast and low-cost liquid–liquid extraction (LLE) method without a derivatization step for quantification of N-benzylpiperazine (BZP) in human plasma using a gas chromatography mass spectrometry (GC-MS) technique was developed. Among the tested extraction agents the best results were obtained with the use of ethyl acetate at pH 12. Optimization of the gas chromatography conditions was conducted including split ratio, temperature gradient of the analytical column and injection volume. The main validation parameters of the proposed method were determined. The extraction recovery was 68% with intra-day and inter-day precision (RSD%) below 5% (n = 3). The limit of detection (LOD) and quantification (LOQ) of BZP was 24.8 and 82.7 ng mL−1, respectively. A linear calibration curve in the range from LOQ to 1000 ng mL−1 was obtained with the use of the selected ion monitoring (SIM) mode with determination coefficient (r2) higher than 0.999. The optimized and validated LLE-GC-MS method was effectively applied for analysis of clinical plasma samples. The proposed sample preparation protocol was found to be a good alternative to the derivatization step and may be applicable in tests for proof of BZP intake in toxicological routine analysis.