Background: Nasal rinsing appears particularly suitable in the management of pregnant women with seasonal allergic rhinitis since no deleterious effects on the fetus are to be expected. However, to date, no studies have specifically investigated this option. Methods: Pregnant women with seasonal allergic rhinitis were randomized to intranasal lavage with hypertonic saline solution 3 times daily (n = 22) versus no local therapy (n = 23) during a 6-week period corresponding to the pollen season. Patients were invited to keep a daily record of rhinitis symptoms (rhinorrea, obstruction, nasal itching and sneezing), to record consumption of oral antihistamine and to undergo rhinomanometry. Results: The rhinitis score was similar at study entry but a statistically significant improvement in this score was observed in the study group during all subsequent weeks (p < 0.001 for weeks 2–6). The mean number of daily antihistamines use per patient per week was significantly reduced at weeks 2, 3 and 6 (p < 0.001, p < 0.001 and p = 0.001, respectively). Baseline rhinomanometry performed at week 1 showed similar nasal resistance in the study and control groups. In contrast, a statistically significant difference emerged in the 2 following evaluations. At week 3, nasal resistance in the study and control groups was 0.96 ± 0.44 and 1.38 ± 0.52 Pa/ml/s, respectively (p = 0.006). At week 6, it was 0.94 ± 0.38 and 1.35 ± 0.60 Pa/ml/s, respectively (p = 0.006). No adverse effect was reported in the active group. Conclusions: Nasal rinsing is a safe and effective treatment option in pregnant women with seasonal allergic rhinitis.