Background: Catechol-O-methyl transferase (COMT) inhibitors are routinely used to manage motor fluctuations in Parkinson's disease (PD). We assessed the effect of opicapone on motor symptom severity in levodopa-treated patients without motor complications. Methods: This was a randomized, double-blind, 24-week, placebo-controlled study of opicapone 50 mg as adjunct to levodopa (NCT04978597). Levodopa-treated patients without motor complications were randomized to 24 weeks of double-blind treatment with adjunct opicapone 50 mg or matching placebo. The primary efficacy endpoint was the mean change from baseline to week 24 in Movement Disorder Society-Unified Parkinson's Disease Rating Scale Part III (MDS-UPDRS-III) total score. Results: A total of 355 patients were randomized (opicapone 50 mg n = 177, placebo n = 178) and 322 (91%) completed the double-blind period. The adjusted mean [95% CI] change from baseline to week 24 in MDS-UPDRS-III subscore was −6.5 [−7.9, −5.2] in the opicapone group versus −4.3 [−5.7, 3.0] in the placebo group resulting in a significant difference of −2.2 [−3.9, −0.5] favoring opicapone (p = 0.010). There was no difference in the incidence of patients who developed motor complications (5.5% with opicapone vs. 9.8% with placebo) and the incidence of adverse events considered related to study medication was similar between groups (opicapone 10.2% vs. placebo 13.5%). Conclusions: Treatment with once-daily adjunct opicapone was well tolerated, improved motor severity, and did not induce the development of motor complications. These results support the clinical usefulness of opicapone in the management of PD patients without motor complications.