BACKGROUND S0597 is a novel glucocorticosteroid that was formulated as an intranasal spray to treat seasonal allergic rhinitis (SAR). In a previous phase 2 study, doses of 100 to 400 μg twice daily were well tolerated and more effective than placebo for improving nasal symptoms induced by grass pollen. OBJECTIVE To assess the clinical efficacy and safety of a once-daily S0597 nasal spray for treatment of SAR induced by ragweed pollen in an environmental exposure unit (EEU). METHODS A single-center, phase 2, randomized, double-blind study in 222 adults with SAR and a positive skin prick test result to short ragweed. Participants underwent ragweed pollen challenge in the EEU at the screening or priming visit and on days 1, 7, and 14 and received 50, 200, or 400 μg of S0597 or placebo in the evening for 13 days. The primary efficacy end point was change in total nasal symptom score (TNSS) from baseline to day 14. RESULTS Improvement in TNSS from baseline to day 14 was statistically significant in all S0597 groups compared with placebo. Least-squares mean differences in change from baseline between active treatment and placebo were 1.18, 1.84, and 1.17 for the 50-, 200-, and 400-μg/d S0597 groups, respectively (P < .05). The 200-μg group demonstrated statistically significant improvements in all TNSS subscales (rhinorrhea, nasal congestion, sneezing, nasal itching) compared with placebo at days 7 and 14. CONCLUSION Treatment with 50 to 400 μg of S0597 once daily was well tolerated and significantly more effective than placebo in relieving nasal symptoms of SAR associated with ragweed pollen. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT01940146.