Background: GSP301 is an investigational fixed-dose combination nasal spray that contains the antihistamine, olopatadine hydrochloride (HCl), and the corticosteroid, mometasone furoate. Objective: To evaluate the efficacy and safety of GSP301 in patients with seasonal allergic rhinitis (SAR). Methods: In this double-blind, randomized, parallel-group study, patients (≥12 years of age) with SAR were equally randomized to intranasal GSP301 (olopatadine 665 μg and mometasone 25 μg), olopatadine HCl (665 μg), mometasone furoate (25 μg), or placebo for 14 days of twice-daily treatment. The primary end point was the mean change from baseline in the average A.M. and P.M. 12-hour reflective Total Nasal Symptom Score (rTNSS) analyzed by using mixed-effect model repeated measures (p < 0.05 indicates statistical significance). Additional assessments included instantaneous TNSS (iTNSS), individual nasal symptoms, reflective Total Ocular Symptom Score (rTOSS) and instantaneous Total Ocular Symptom Score (iTOSS), onset of action, Physician-assessed Nasal Symptom Score (PNSS), quality of life, and adverse events (AE). Results: A total of 1180 patients were randomized. Over 14 days of treatment, GSP301 significantly improved average A.M. and P.M. rTNSS versus placebo (least squares mean difference -0.98 [95% confidence interval, -1.38 to -0.57]; p < 0.001) and versus olopatadine (p = 0.003), and approached statistical significance versus mometasone (p = 0.059). GSP301 also significantly improved average A.M. and P.M. iTNSS versus placebo and both monotherapies (p < 0.05, all). Further, GSP301 significantly improved individual nasal symptoms, overall ocular symptoms (rTOSS and iTOSS), and overall quality of life versus placebo (p < 0.01, all). Onset of action for GSP301 was observed within 15 minutes and was maintained at all subsequent time points assessed. Results for the PNSS also were significant for GSP301 versus placebo (p < 0.001). The percentages of patients with treatment-emergent AEs treated with GSP301, olopatadine, mometasone, and placebo were 12.9, 12.5, 7.1, and 9.4%, respectively. Conclusion: GSP301 was efficacious and well tolerated for the treatment of SAR symptoms compared with placebo, with a rapid onset of action of 15 minutes in patients ≥12 years of age.Clinical trial NCT02631551, <ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.clinicaltrials.gov">www.clinicaltrials.gov</ext-link>.