Importance: Emergency department (ED)-initiated buprenorphine for the treatment of opioid use disorder (OUD) is underused. Objective: To evaluate whether provision of ED-initiated buprenorphine with referral for OUD increased after implementation facilitation (IF), an educational and implementation strategy. Design, Setting, and Participants: This multisite hybrid type 3 effectiveness-implementation nonrandomized trial compared grand rounds with IF, with pre-post 12-month baseline and IF evaluation periods, at 4 academic EDs. The study was conducted from April 1, 2017, to November 30, 2020. Participants were ED and community clinicians treating patients with OUD and observational cohorts of ED patients with untreated OUD. Data were analyzed from July 16, 2021, to July 14, 2022. Exposure: A 60-minute in-person grand rounds was compared with IF, a multicomponent facilitation strategy that engaged local champions, developed protocols, and provided learning collaboratives and performance feedback. Main Outcomes and Measures: The primary outcomes were the rate of patients in the observational cohorts who received ED-initiated buprenorphine with referral for OUD treatment (primary implementation outcome) and the rate of patients engaged in OUD treatment at 30 days after enrollment (effectiveness outcome). Additional implementation outcomes included the numbers of ED clinicians with an X-waiver to prescribe buprenorphine and ED visits with buprenorphine administered or prescribed and naloxone dispensed or prescribed. Results: A total of 394 patients were enrolled during the baseline evaluation period and 362 patients were enrolled during the IF evaluation period across all sites, for a total of 756 patients (540 [71.4%] male; mean [SD] age, 39.3 [11.7] years), with 223 Black patients (29.5%) and 394 White patients (52.1%). The cohort included 420 patients (55.6%) who were unemployed, and 431 patients (57.0%) reported unstable housing. Two patients (0.5%) received ED-initiated buprenorphine during the baseline period, compared with 53 patients (14.6%) during the IF evaluation period (P <.001). Forty patients (10.2%) were engaged with OUD treatment during the baseline period, compared with 59 patients (16.3%) during the IF evaluation period (P =.01). Patients in the IF evaluation period who received ED-initiated buprenorphine were more likely to be in treatment at 30 days (19 of 53 patients [35.8%]) than those who did not 40 of 309 patients (12.9%; P <.001). Additionally, there were increases in the numbers of ED clinicians with an X-waiver (from 11 to 196 clinicians) and ED visits with provision of buprenorphine (from 259 to 1256 visits) and naloxone (from 535 to 1091 visits). Conclusions and Relevance: In this multicenter effectiveness-implementation nonrandomized trial, rates of ED-initiated buprenorphine and engagement in OUD treatment were higher in the IF period, especially among patients who received ED-initiated buprenorphine. Trial Registration: ClinicalTrials.gov Identifier: NCT03023930.