Background Current guidelines recommend administering dual antiplatelet therapy (DAPT) for 12 months to patients with acute coronary syndromes (ACS) and without contraindications after drug-eluting stent (DES) implantation. A recent study reported that 3 months of DAPT followed by ticagrelor monotherapy is effective and safe in ACS patients undergoing DES implantation compared with the standard duration of DAPT. However, it is unclear whether antiplatelet monotherapy with ticagrelor alone versus ticagrelor plus aspirin reduces the incidence of clinically relevant bleeding without increasing the risk of major adverse cardiovascular and cerebrovascular events (MACCEs) in ACS patients undergoing percutaneous coronar y inter vention (PCI) with DES implantation guided by either intravascular ultrasound (IVUS) or angiography who have completed a 1-month course of DAPT with aspirin plus ticagrelor. Methods The IVUS-ACS and ULTIMATE-DAPT is a prospective, multicenter, randomized, controlled trial designed to determine (1) whether IVUS-guided versus angiography-guided DES implantation in patients with ACS reduces the risk of target vessel failure (TVF) at 12 months and (2) whether ticagrelor alone versus ticagrelor plus aspirin reduces the risk of clinically relevant bleeding without increasing the risk of MACCE 1-12 months after the index PCI in ACS patients undergoing DES implantation guided by either IVUS or angiography. This study will enroll 3486 ACS patients eligible for DES implantation, as confirmed by angiographic studies. The patients who meet the inclusion criteria and none of the exclusion criteria will be randomly assigned in a 1:1 fashion to the IVUS- or angiography-guided group (first randomization). All enrolled patients will complete a 1-month course of DAPT with aspirin plus ticagrelor after the index PCI. Patients with no MACCEs or major bleeding (>= Bleeding Academic Research Consortium (BARC) 3b) within 30 days will be randomized in a 1:1 fashion to either the ticagrelor plus matching placebo (SAPT)group or ticagrelor plus aspirin (DAPT)group for an additional 11 months (second randomization). The primary endpoint of the IVUS-ACS trial is TVF at 12 months, including cardiac death, target vessel myocardial infarction (TVMI), or clinically driven target vessel revascularization (CD-TVR). The primary superiority endpoint of the ULTIMATE-DAPT trial is clinically relevant bleeding, defined as BARC Types 2, 3, or 5 bleeding, and the primary non-inferiority endpoint of the ULTIMATE-DAPT trial is MACCE, defined as cardiac death, myocardial infarction, ischemic stroke, CD-TVR, or definite stent thrombosis occurring 1-12 months in the second randomized population. Conclusion The IVUS-ACS and ULTIMATE-DAPT trial is designed to test the efficacy and safety of 2 different antiplatelet strategies in ACS patients undergoing PCI with DES implantation guided by either IVUS or angiography. This study will provide novel insights into the optimal DAPT duration in ACS patients undergoing PCI and provide evidence on the clinical benefits of IVUS-guided PCI in ACS patients.