Background: Safety and efficacy of GSP301 nasal spray, an investigational fixed-dose combination of olopatadine hydrochloride and mometasone furoate, was established in three large, 2-week seasonal allergic rhinitis studies. Objective: To evaluate long-term (52 weeks) safety and efficacy of GSP301 in patients with perennial allergic rhinitis (PAR). Methods: In this randomized, double-blind, parallel-group study, 601 patients (ages ≥ 12 years) with PAR were randomized 4:1:1 to twice-daily GSP301 (olopatadine 665 μg and mometasone 25 μg [pH 3.7]) or two GSP301 vehicle formulations (placebo pH 3.7 or 7.0). Safety (primary end point) was monitored through adverse events (AE), laboratory assessments, vital signs, and physical examinations at weeks 30 and 52. The change from baseline in the average A.M. reflective Total Nasal Symptom Score (rTNSS) and instantaneous Total Nasal Symptom Score (iTNSS), Physician-assessed Nasal Symptom Scores (PNSS), and quality of life were assessed for GSP301 versus placebo pH 3.7 (p < 0.05 was considered statistically significant). Results: At week 52, treatment-emergent AEs (TEAE) occurred in 51.7, 41.4, and 53.5% of patients in the GSP301, placebo pH 3.7 and placebo 7.0 groups, respectively. No clinically meaningful differences were observed in TEAE incidences or other safety assessments across treatments. At weeks 6 and 30, GSP301 provided significant and clinically meaningful improvements in average rTNSS and iTNSS versus placebo pH 3.7 (p < 0.01, all comparisons). Similarly, at week 52, GSP301 provided significant and clinically meaningful improvements in rTNSS (least-squares mean difference -0.91 [95% confidence interval {CI}, -1.35 to -0.47]; p < 0.001), and iTNSS (least-squares mean difference -0.75 [95% CI, -1.17 to -0.33]; p < 0.001) versus placebo pH 3.7, with significant improvements in each individual symptom (p < 0.05, all comparisons). PNSS and quality of life were significantly improved versus placebo pH 3.7 at weeks 6 and 30 (p < 0.05, all comparisons), but these greater improvements did not reach statistical significance at week 52 (PNSS, p = 0.552; quality of life, p = 0.790). Conclusion: Twice-daily GSP301 was well tolerated and provided statistically significant and clinically meaningful improvements in PAR nasal symptoms versus placebo over 52 weeks and demonstrated a favorable safety profile and efficacy.Clinical trial NCT02709538, <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>.