Among the new psychoactive substances (NPS) that 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‐phenyl‐N‐[1‐(2‐phenethyl)piperidin‐4‐yl]prop‐2‐enamide (acrylfentanyl), was identified in powder from a seized capsule found at a forensic psychiatric ward in Denmark. Gas chromatography with mass spectrometry (GC‐MS) identified a precursor to synthetic fentanyls, N‐phenyl‐1‐(2‐phenylethyl)piperidin‐4‐amine; however, the precursor 1‐(2‐phenethyl)piperidin‐4‐one, was not detected. Analysis of the electron impact mass spectrum of the main, unknown chromatographic peak (GC) tentatively identified an acryloyl analogue of fentanyl. Further analyses by quadrupole time‐of‐flight high resolution mass spectrometry (QTOF‐MS), matrix‐assisted laser ionization Orbitrap mass spectrometry (MALDI‐Orbitrap‐MS), nuclear magnetic resonance spectroscopy (NMR), and infra‐red spectroscopy (IR) confirmed the presence of acrylfentanyl (also known as acryloylfentanyl). Quantitative analysis with liquid chromatography and triple quadrupole mass spectrometry (LC‐MS/MS) determined the content of acrylfentanyl in the powder, equal to 88.3 mass‐% acrylfentanyl hydrochloride. An impurity observed by NMR was identified as triethylamine hydrochloride. Acrylfentanyl is sold on the Internet as a ‘research chemical’. Like other synthetic fentanyls, such as acetylfentanyl, it poses a serious risk of fatal intoxication. Copyright © 2016 The Authors. Drug Testing and Analysis Published by John Wiley & Sons Ltd.