Background: Patients with head and neck cancer who are receiving radiotherapy can develop aspiration pneumonia. Determination of the incidence of aspiration pneumonia and the associated risk factors could facilitate the identification of high-risk patients. Methods: In this retrospective study, we determined the incidence of aspiration pneumonia in 357 patients receiving radiotherapy along with oral care for head and neck cancer. We also performed univariate and multivariable logistic regression analyses to investigate the risk factors for this complication. Results: The incidence of aspiration pneumonia was 17.6%. Hypopharyngeal cancer, grade 3 oral mucositis, and nasogastric tube feeding were independent risk factors. Moreover, the development of aspiration pneumonia was one of the major effects on the discontinuation of radiotherapy. Conclusion: Approximately, one-sixth of the patients developed aspiration pneumonia despite appropriate oral care during radiotherapy for head and neck cancer. Aspiration pneumonia during radiotherapy could adversely affect head and neck cancer management.