IMPORTANCE Ubrogepant is an oral calcitonin gene-related peptide receptor antagonist under investigation for acute treatment of migraine. OBJECTIVE To evaluate the efficacy and tolerability of ubrogepant compared with placebo for acute treatment of a single migraine attack. DESIGN, SETTING, AND PARTICIPANTS Phase 3, multicenter, randomized, double-blind, placebo-controlled, single-attack, clinical trial (ACHIEVE II) conducted in the United States (99 primary care and research clinics; August 26, 2016-February 26, 2018). Participants were adults with migraine with or without aura experiencing 2 to 8 migraine attacks per month. INTERVENTIONS Ubrogepant 50 mg (n = 562), ubrogepant 25 mg (n = 561), or placebo (n = 563) for a migraine attack of moderate or severe pain intensity. MAIN OUTCOMES AND MEASURES Co-primary efficacy outcomes were pain freedom and absence of the participant-designated most bothersome migraine-associated symptom (among photophobia, phonophobia, and nausea) at 2 hours after taking the medication. RESULTS Among 1686 randomized participants, 1465 received study treatment (safety population; mean age, 41.5 years; 90% female); 1355 of 1465 (92.5%) were evaluable for efficacy. Pain freedom at 2 hours was reported by 101 of 464 participants (21.8%) in the ubrogepant 50-mg group, 90 of 435 (20.7%) in the ubrogepant 25-mg group, and 65 of 456 (14.3%) in the placebo group (absolute difference for 50 mg vs placebo, 7.5%; 95% CI, 2.6%-12.5%; P = .01; 25 mg vs placebo, 6.4%; 95% CI, 1.5%-11.5%; P = .03). Absence of the most bothersome associated symptom at 2 hours was reported by 180 of 463 participants (38.9%) in the ubrogepant 50-mg group, 148 of 434 (34.1%) in the ubrogepant 25-mg group, and 125 of 456 (27.4%) in the placebo group (absolute difference for 50 mg vs placebo, 11.5%; 95% CI, 5.4%-17.5%; P = .01; 25 mg vs placebo, 6.7%; 95% CI, 0.6%-12.7%; P = .07). The most common adverse events within 48 hours of any dose were nausea (50 mg, 10 of 488 [2.0%]; 25 mg, 12 of 478 [2.5%]; and placebo, 10 of 499 [2.0%]) and dizziness (50 mg, 7 of 488 [1.4%]; 25 mg, 10 of 478 [2.1%]; placebo, 8 of 499 [1.6%]). CONCLUSIONS AND RELEVANCE Among adults with migraine, acute treatment with ubrogepant compared with placebo led to significantly greater rates of pain freedom at 2 hours with 50-mg and 25-mg doses, and absence of the most bothersome migraine-associated symptom at 2 hours only with the 50-mg dose. Further research is needed to assess the effectiveness of ubrogepant against other acute treatments for migraine and to evaluate the long-term safety of ubrogepant among unselected patient populations.