Background and Objective Viloxazine ER (extended-release capsules; Qelbree®) is a nonstimulant medication that has been approved by the United States Food and Drug Administration (FDA) for treatment of attention-deficit/hyperactivity disorder (ADHD) in children (> 6 years old) and adults. This phase 3 open-label extension to a pivotal phase 3, double-blind trial evaluated the long-term safety and continued efficacy of viloxazine ER in adults with ADHD. Methods This was a multicenter, flexible-dose, open-label extension to a phase III, double-blind, placebo-controlled trial (NCT04016779). Viloxazine ER was initiated at 200 mg/day and adjusted (between 200 and 600 mg/day) to achieve optimal efficacy and tolerability. Trial enrollment was halted temporarily (24 March 2020 to 23 July 2020) due to the coronavirus disease 2019 (COVID-19) pandemic. Participants completing double-blind treatment during that time were offered delayed enrollment upon trial requalification. Safety outcomes were the primary objectives. Secondary objectives were efficacy outcomes, including the ADHD Investigator Symptom Rating Scale (AISRS), and were assessed relative to double-blind baseline (or trial re-entry baseline for those whose enrollment was delayed by the COVID-19 pandemic). Results Overall, 159 participants (133 immediate and 26 delayed rollover) received viloxazine ER, with a mean exposure of 265 ± 254.9 days. Adverse events (AEs) included (> 10% incidence) insomnia (13.8%), nausea (13.8%), headache (10.7%), and fatigue (10.1%). AEs led to discontinuation for 17.6% of participants [most commonly insomnia (2.5%), nausea (2.5%), and fatigue (1.9%)]. AISRS total score [baseline mean ± standard deviation (SD): 37.9 ± 6.3] improved by the first follow-up visit (−11.4 ± 9.5; week 2) with continued improvement at subsequent visits (last on-study visit: −18.2 ± 11.54). Similar patterns of improvement were seen for other measures of efficacy, including quality of life and executive function. Following initial dose optimization, most participants (73%) used viloxazine ER doses ≥ 400 mg/day, with 36% using doses of 600 mg/day. Conclusions Long-term viloxazine ER use was well tolerated, with no new long-term safety findings. Improvements in ADHD symptoms and associated measures were sustained throughout trial participation. In total, 73% percent of adult participants in this long-term study used viloxazine ER doses of 400 mg or more during maintenance treatment. Clinical Trial Registration Clinicaltrials.gov Identifier: NCT04143217.