Nonerosive reflux disease (NERD) is the most common form of gastroesophageal reflux disease. Patients with NERD have a lower response rate to proton pump inhibitors (PPIs) than patients with erosive esophagitis when gauged from relief of heartburn. Sodium alginate decreases the acidity of refluxate and protects the esophageal mucosa. However, whether the addition of sodium alginate to PPI therapy can improve NERD symptoms remains unknown. Accordingly, the aim of this study was to evaluate the efficacy of adding sodium alginate to basal PPI therapy for NERD. Patients who had experienced heartburn on at least 2 days per week during the 1-month period before entering the study and had no endoscopic mucosal breaks (grade M or N according to Hoshihara's modification of the Los Angeles classification) were randomized to one of two treatments for 4 weeks: omeprazole (20 mg once daily) plus sodium alginate (30 mL four times a day) (group A) or omeprazole (20 mg once daily) alone (group B). Eighty-seven patients were enrolled, and 76 patients were randomly assigned to group A (n = 36) or group B (n = 40). Complete resolution of heartburn for at least 7 consecutive days by the end of treatment was significantly more common in group A (56.7%) than in group B (25.7%). One patient from group A had mild drug-related diarrhea that was not clinically serious. In conclusion, omeprazole combined with sodium alginate was better than omeprazole alone in Japanese patients with NERD.