Background: Drug-free viscous nasal applications have been shown to reduce nasal symptoms in individuals with seasonal allergic rhinitis (SAR). Nascum®-Plus (NP), a commercially available thixotropic gel, has been designed to reduce dryness and soreness of the nasal mucosa and prevent the absorption of small particles. Objectives: The aim of this study was to assess the efficacy of single-dose NP in treating nasal symptoms and secretion during challenge in an allergen challenge chamber (ACC). Furthermore, the effect of this treatment on biomarkers and immune cells of the allergic cascade were measured. Methods: This open-label, cross-over, sequence-randomized, monocentric trial randomized 18 adults with SAR and a positive skin prick test reaction to Dactylis glomerata pollen to receive NP or no treatment during two 4-h ACC sessions 3 weeks apart. On Day 1, 9 subjects were challenged for 4 h with treatment, the other 9 without treatment, and vice versa on Day 22. Nasal lavage fluid and nasal filter eluate samples were obtained pre, 2, and 18 h post challenge in the ACC. Results: NP significantly reduced nasal symptoms, assessed by total nasal symptom score (p < 0.001), and minimized nasal secretion (p = 0.047), while no significant effect on biomarkers and immune cells in the nasal fluid was observed. The treatment was safe and well-tolerated. Conclusions: The physical barrier built by NP nasal gel can be safely applied in patients with allergic rhinitis. It reduces allergic nasal symptoms and secretion, but application of a single dose does not affect local inflammatory biomarkers.