Most recently, the synthetic opioid U-47700 has emerged on the illicit drug market and is sold on the Internet as a "research chemical". Its structure is closely related to the synthetic opioid AH-7921. U-47700 is a μ-opioid receptor agonist with a potency of approximately 7.5 times that of morphine. In this study, postmortem concentrations of U-47700 are presented in 26 fatalities which occurred between April 2016 and August 2017 in the southern part of Germany. In 18 of these cases, quantitative analyses of U-47700 were carried out in femoral blood, heart blood, liver, urine, vitreous humor, pericardial fluid, and gastric content. In five cases, concentrations of U-47700 were determined in femoral blood, whereas in one case, the concentration of U-47700 was analyzed in heart blood. Due to advanced putrefaction, the analysis of U-47700 could only be performed in putrefaction fluid in two cases. Quantification of U-47700 was carried out using liquid chromatography-quadrupole-time-of-flight mass spectrometry (LC-QTOF-MS) with electrospray ionization operated in positive mode. The median femoral blood concentration of U-47700 (n = 23) was 610 ng/mL (range: 27-2200 ng/mL). Except for one female, all decedents were male and aged between 23 and 56 years (mean age: 34 years). In all fatalities, the cause of death was attributed to an intoxication with U-47700 either alone or in combination with other psychoactive substances. In 15 of the 26 cases, there was a combined use of U-47700 with other new psychoactive substances (NPS). Therefore, not only new synthetic opioids but also additional NPS including synthetic cannabinoids, new stimulant drugs, and designer benzodiazepines should be included in the routine toxicological screening methods.