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Abstract Purpose Carrageenan-containing nasal sprays are known to alleviate symptoms of common cold and allergic symptoms by building a barrier against airborne intruders. The objective of this study was to develop a hyperosmolar nasal spray with barrier-forming properties and to demonstrate its decongestant effect in the context of allergic rhinitis. Methods The efficacy of the nasal spray components was first demonstrated in vitro by a virus replication inhibition, water absorption, and barrier assay. Clinical efficacy was assessed in a randomized, controlled, crossover trial, where adults with a history of severe seasonal allergic rhinitis were exposed to grass pollen allergens under controlled conditions for a total of 6 hours. Participants received either the carrageenan- and sorbitol containing nasal spray (CS) or saline solution (SS) after 1h45min of allergen exposure. After one week, participants repeated the exposure, receiving the treatment (CS or SS) they had not received before. The primary efficacy endpoint was the mean change in ’Nasal Congestion Symptom Score’ (NCSS) during the allergen exposure. Secondary efficacy endpoints were nasal airflow, nasal secretion, total nasal symptom score (TNSS), total ocular symptom score (TOSS) and total respiratory symptom score (TRSS). Results Preclinical assays showed virus-blocking, barrier building and water withdrawing properties of the CS components. In the clinical study, a total of 46 participants were screened, 41 were randomized and 39 completed the study. There was no significant difference in mean NCSS change from pre- to post-treatment between CS and SS (mean difference of 0.02 [95% CI -0.19; 0.24] during the first 2 hours after treatment) when analyzed by intention-to-treat. However, nasal airflow increased over time after treatment with CS, while it declined after SS, leading to a growing difference in airflow between CS- and SS-treated participants (p=0.039 at 6:00h). The anterior nasal airflow increased after treatment in 23/38 (61%) of the CS treated participants, compared to only 13/38 (34%) of the SS treated participants (p=0.024). The mean nasal secretion over 2-6 h was reduced by 1.00 g or -25% after CS (p=0.003) compared to pre-treatment, while it was reduced by only -0.50 g after SS (p=0.137). No significant differences in TNSS, TOSS and TRSS were observed between CS and SS treatments. Conclusion CS builds a barrier at the mucosa against viruses and dust and is safe and effective in alleviating nasal congestion, nasal airflow and nasal secretion in adults with grass pollen allergy. Trial registration NCT04532762