Purpose We investigated the clinical outcome of sodium alginate treatment in radiation-induced pharyngeal mucositis (RIPM) after neck irradiation. Materials and methods The study population included 32 patients (11 lung cancer, 10 breast cancer, 7 head and neck cancer, and 4 other primary lesions) who underwent neck external beam radiotherapy at the authors’ institution between June 2006 and 2016. The patients received 5% sodium alginate solution orally for RIPM. Those who were followed up for less than 2 months or did not receive 5% sodium alginate were excluded from this retrospective study. RIPM was graded weekly as an acute toxicity according to the Common Terminology Criteria for Adverse Events (CTCAE), version 4. The administration of 10-15 ml of sodium alginate before each meal was continued until the radiotherapy was completed and after resolution of odynophagia. The efficacy of sodium alginate was assessed by two radiation oncologists as follows: Grade I, ineffective; grade II, moderately effective; grade III, very effective. When sodium alginate was ineffective, other analgesics, such as nonsteroidal anti-inflammatory drugs (NSAIDS) or opioids, were added. Relationships between the presence/absence of additional analgesics and the radiation dose were investigated. Results The median duration from the start of irradiation to sodium alginate administration was 15 days (range, 5–36 days). RIPM improved in 29/32 patients (grade: II, n = 22; III, n = 7). Three patients showed no improvement. No sodium alginate-related toxicities occurred. Additional analgesics were required in 5/32 patients. The radiation dose in these 5 patients was significantly higher than that in the sodium alginate-alone group (63.6 ± 7.8 Gy vs 48.3 ± 14.8Gy, P = 0.02). Patients who received > 50 Gy tended to require additional analgesics more frequently than those who received ≤50Gy (P = 0.10). Conclusions The concurrent administration of sodium alginate and neck irradiation was feasible and tolerable without obvious toxicities. Under certain conditions sodium alginate could be a promising alternative to NSAIDs and opioids in RIPM. The results justify further prospective evaluations with detailed treatment protocols to clarify whether sodium alginate can improve RIPM.