Objective: To assess the utility of the novel patient-identified (PI) most bothersome symptom (MBS) measure from PROMISE-2, a phase 3 trial of eptinezumab for the preventive treatment of chronic migraine. Background: Relief of bothersome migraine symptoms can influence satisfaction with treatment and therapeutic persistence. Understanding the impact of preventive treatment on a PI-MBS could improve clinical decision-making. Methods: In PROMISE-2, patients with chronic migraine received eptinezumab 100, 300 mg, or placebo administered intravenously every 12 weeks for up to 2 doses (n = 1072). PI-MBS was an exploratory outcome requiring each patient to self-report their MBS in response to an open-ended question. At baseline and week 12, patients rated overall improvement in PI-MBS. The relationships among PI-MBS at week 12 and change in monthly migraine days (MMDs) from baseline to month 3 (weeks 9–12), Patient Global Impression of Change at week 12, and changes from baseline to week 12 in the 6-item Headache Impact Test total, EuroQol 5-dimensions 5-levels visual analog scale, and 36-item Short-Form Health Survey component scores were assessed. Results: Treatment groups had similar baseline characteristics and reported a total of 23 unique PI-MBS, most commonly light sensitivity (200/1072, 18.7%), nausea/vomiting (162/1072, 15.1%), and pain with activity (147/1072, 13.7%). Improvements in PI-MBS at week 12 correlated with changes in MMDs (ρ = −0.49; p < 0.0001) and other patient-reported outcomes. Controlling for changes in MMDs, PI-MBS improvement predicted other patient-reported outcomes in expected directions. The magnitude of the standardized mean differences between placebo and active treatment for PI-MBS were 0.31 (p < 0.0001 vs. placebo) and 0.54 (p < 0.0001 vs. placebo) for eptinezumab 100 and 300 mg, respectively. Conclusions: Improvement in PI-MBS at week 12 was associated with improvement in other patient-reported outcome measures, and PI-MBS may be an important patient-centered measure of treatment benefits in patients with chronic migraine.