Importance: After disparate results from observational and small randomized studies, the COMPLETE trial demonstrated superiority of multivessel (MV) percutaneous coronary intervention (PCI) over culprit-only PCI for ST-elevation myocardial infarction (STEMI). Objective: To describe temporal trends and institutional variation of MV PCI use for STEMI in the United States to inform how new evidence may influence clinical practice. Design, Setting, and Participants: This cohort study included STEMI admissions involving MV disease from 1598 institutions in the National Cardiovascular Data Registry CathPCI Registry from the third quarter of 2009 to the first quarter of 2018. An MV PCI was defined as a PCI to a nonculprit lesion within 45 days of the index procedure. Exposures: Multivessel PCI, defined as placement of coronary stents in 2 or more major epicardial vessels or the staged placement of 1 or more coronary stents in a major epicardial vessel distinct from the index culprit vessel, within 45 days of the index PCI. Main Outcomes and Measures: Outcomes included the proportional use of MV PCI among STEMI admissions with MV disease, and the timing of MV PCI (an index procedure, a staged procedure during index hospitalization, or a postdischarge procedure within 45 days). Results: Among 359879 admissions with STEMI and MV disease, MV PCI was performed in 38.5% (n = 138380; mean [SD] age of patients, 62.3 [12.3] years; 102266 men [73.9%]) within 45 days. Of those receiving MV PCIs, 30.8% (n = 42629) had a procedure performed during the index procedure, 31.6% (n = 43696) as a staged procedure during the index hospitalization, and 37.6% (n = 52055) within 45 days of discharge. Complete revascularization of all diseased arteries was performed in 76.2% (n = 105389). From the third quarter of 2009 to the second quarter of 2013, MV PCI use declined by 10%, from 42.7% (3230 of 7572 cases) to a nadir of 32.7% (3386 of 10342 cases), followed by an increase to 44.0% (5062 of 11497 cases) by the fourth quarter of 2017. During this time, there was a 13.6% decline in use of postdischarge staged MV PCI (from 23.4% of STEMI cases [1772 of 7572 cases] in the third quarter of 2009 to 9.9% [1094 of 11171 cases] in the fourth quarter of 2014) and an 12.5% increase in MV PCI performed during the index admission (from 19.3% [1458 of 7572 cases] in the third quarter of 2009 to 31.8% [3557 of 11171 cases] in the first quarter of 2018). Multivessel PCI use varied substantially across institutions, with a median use of 37.9% (interquartile range, 30.0%-46.5%). Conclusions and Relevance: In this large, nationwide analysis, MV PCI use for patients with STEMI has been increasing through early 2018 but was used in the minority of patients and with wide variability across US institutions. The adoption of new trial results into guidelines and practice may further promote the growth of MV PCI..