Background: Patients suffering from locoregionally advanced nasopharyngeal carcinoma (NPC) commonly develop disease recurrence, despite a high rate of complete clinical remission after standard of care (concurrent cisplatin-radiotherapy, with or without induction chemotherapy). The benefit of additional adjuvant chemotherapy remains unclear. Methods: Patients with high-risk locoregionally advanced NPC (stage III to IVA, excluding T3-4N0 and T3N1), and with no locoregional disease or distant metastasis after definitive chemoradiotherapy, were eligible. They were randomly assigned (1:1) within 12 to 16 weeks after the last radiation dose to receive either capecitabine at a dose of 650 mg/m² twice daily for 1 year (metronomic capecitabine group) or observation (standard-therapy group). The primary end point was recurrence-free survival (RFS). The calculated sample size was 201 per group, with an 80% power (two-sided α 0.05) to detect a target hazard ratio (HR) of 0.52. Results: A total of 406 patients underwent randomization, comprising 204 in the metronomic capecitabine group and 202 in the standard-therapy group. After a median follow-up of 36 months (corresponding to 43 months when calculated from the start of standard therapy), the estimated 3-year RFS was 85.9% in the metronomic capecitabine group, as compared with 76.5% in the standard-therapy group (intention-to-treat population; HR 0.51, 95% confidence interval 0.32–0.81; P = 0.003). The incidence of grade 3 adverse events was 17.4% in the metronomic capecitabine group and 5.5% in the standard-therapy group; hand-foot syndrome was the most common adverse event related to capecitabine (9.0%). One grade 4 neutropenia occurred in the metronomic capecitabine group. Neither group sufferd from treatment-related deaths. During treatment, there was no clinically meaningful deterioration of health-related quality of life associated with the use of metronomic adjuvant capecitabine. Conclusions: The addition of metronomic capecitabine as adjuvant therapy to chemoradiotherapy significantly improved RFS in locoregionally advanced NPC, with a manageable safety profile and no compromise to quality of life. Clinical trial information: NCT02958111