Among the new psychoactive substances (NPS), which have recently emerged on the market, many of the new synthetic opioids have shown to be particularly harmful. A new synthetic analogue of fentanyl, N-(1-phenethylpiperidin-4-yl)-N-phenylacrylamide (acrylfentanyl), was identified in a seized capsule found at a forensic psychiatric ward in Denmark. Gas chromatography with mass spectrometry (GC-MS) identified the synthetic precursor N-phenyl-1-(2-phenylethyl)piperidin-4- amine (ANPP). The precursor used to synthesise ANPP, 1-(2-phenethyl)-4-piperidon (NPP), was not detected. Analysis of the electron impact mass spectrum of the main, unknown chromatographic peak (GC) tentatively identified an acryloyl derivative of fentanyl. Further analysis by quadrupole time-of-flight, high resolution mass spectrometry (QTOF-MS), matrix-assisted laser ionisation time-of-flight mass spectrometry (MALDI-TOF) and nuclear magnetic resonance spectroscopy (NMR) confirmed the presence of acrylfentanyl (also known as acryloylfentanyl). Acrylfentanyl is sold on the Internet as a ‘research chemical’. It is very potent and poses a serious risk of overdose.