Amphetamine, methamphetamine, phentermine, 3,4-methylenedioxyamphetamine (MDA), 3,4-methylenedioxymethamphetamine (MDMA), and 3,4-methylenedioxy-N-ethylamphetamine (MDEA) are the most popular amphetamine-type stimulants. The use of these substances is a serious societal problem worldwide. In this study, a method based on gas chromatography–tandem mass spectrometry (GC–MS/MS) with simple and rapid liquid-liquid extraction (LLE) and derivatization was developed and validated for the simultaneous determination of the six aforementioned amphetamine derivatives in blood and urine. The detection of all compounds was based on multiple reaction monitoring (MRM) transitions. The most important advantage of the method is the minimal sample volume (as low as 200 μL) required for the extraction procedure. The validation parameters, i.e., the recovery (90.5–104%), inter-day accuracy (94.2–109.1%) and precision (0.5–5.8%), showed the repeatability and sensitivity of the method for both matrices and indicated that the proposed procedure fulfils internationally established acceptance criteria for bioanalytical methods The procedure was successfully applied to the analysis of real blood and urine samples examined in 22 forensic toxicological cases. To the best of our knowledge, this is the first work presenting the use of GC–MS/MS for the determination of amphetamine-type stimulants in blood and urine. In view of the low limits of detection (0.09–0.81 ng/mL), limits of quantification (0.26–2.4 ng/mL), and high selectivity, the procedure can be applied for drug monitoring in both fatal and non-fatal intoxication cases in routine toxicology analysis.