N-(1-Phenethylpiperidin-4-yl)-N-phenylacrylamide, or acryloylfentanyl (acrylfentanyl), is a synthetic opioid and a close structural analogue of fentanyl, which is widely used in medicine as an adjunct to general anaesthesia during surgery and for pain management. Until recently, acryloylfentanyl was known only from the scientific literature, but in 2016 this non-controlled substance became available on the illicit drug market as a powder and nasal spray in Europe and the USA. By the end of 2016, detection of acryloylfentanyl in six European countries, including 47 deaths associated with the drug, had been reported to the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) through the European Union Early Warning System, which is a part of the system designed to identify and respond to the appearance of new psychoactive substances that may pose potential public health risks similar to drugs controlled under the United Nations drug control conventions. Herein we review what is known about this potent narcotic opioid. In addition to describing its chemical properties and the synthetic routes, analytical methodologies for the identification of the substance, as well as the limited information on the biological properties, including in vitro and in vivo pharmacological studies with the substance, are summarised. Analytically confirmed acute intoxications show that the signs and symptoms of acryloylfentanyl poisoning correspond to the opioid overdose triad of decreased consciousness, miosis and respiratory depression. Importantly, naloxone works as an antidote in life-threatening poisoning. The major human urinary metabolites identified in fatal overdose cases were nor-acryloylfentanyl, as well as mono- and dihydroxylated derivatives and their conjugates.