Background: Morphine reduces absorption and delays action onset of potent oral P2Y₁₂ receptor inhibitors in patients with ST-segment elevation myocardial infarction (STEMI). We sought to determine the differential effects of fentanyl compared to morphine on the pharmacodynamics and pharmacokinetics of ticagrelor in STEMI patients undergoing primary percutaneous coronary intervention (PCI). Methods: PERSEUS (NCT02531165) was a prospective, single-center, open-label, randomized controlled study. Patients with STEMI who required analgesia were randomly assigned in a 1:1 ratio to treatment with intravenous fentanyl or morphine after ticagrelor loading dose (LD) administration. The primary endpoint was platelet reactivity at 2 hours after ticagrelor LD assessed by P2Y₁₂ reaction units (PRU). Results: The study was prematurely stopped in June 2017 after enrolment of 38 out of 56 planned patients. PRU at 2 hours following ticagrelor LD was 173.3 ± 89.7 in the fentanyl group and 210.3 ± 76.4 in the morphine group (p = 0.179). At 4 hours, PRU was significantly lower among patients treated with fentanyl as compared to those treated with morphine (90.1 ± 97.4 vs. 168.0 ± 72.2; p = 0.011). Maximal plasma concentrations of ticagrelor and its active metabolite AR-C124910XX tended to be delayed and numerically lower among patients treated with morphine compared to fentanyl. Total exposures to ticagrelor and AR-C124910XX within 6 hours after ticagrelor LD were numerically greater among patients treated with fentanyl compared to those treated with morphine. Conclusions: In patients with STEMI undergoing primary PCI, fentanyl did not improve platelet inhibition at 2 hours after ticagrelor pre-treatment compared with morphine. Fentanyl may, however, accelerate ticagrelor absorption and increase platelet inhibition at 4 hours compared to morphine.