Background: A few studies suggest that self-efficacy pertaining to swallowing ability can lead to dysphagia. Therefore, this cross-sectional study verified the relationship between oral health-related self-efficacy and dysphagia severity during cancer treatment. Methods: Participants included patients undergoing treatment for cancer at Shimane University Hospital, Shimane, Japan, and those receiving outpatient treatment at the hospital’s Oral Care Center between August 2018 and April 2019. In all, 203 participants enrolled in the study and completed the Functional Oral Intake Scale (FOIS), Self-efficacy Scale for Advanced Cancer (SEAC), and Oral Health-related Self-Efficacy Scale for Patients with Cancer (OSEC). Results: A multivariate analysis showed a statistically significant correlation between the low FOIS score category and the SEAC subscales of Activities of Daily Living Self-efficacy (ADE) (odds ratio 1.04, 95% confidence interval [CI] 1.00–1.07) and Symptom Coping Self-efficacy (SCE) (odds ratio 0.61, 95% [CI] 0.42–0.88). Based on the Jonckheere-Terpstra test, the SEAC and OSEC scores tended to increase as the FOIS category progressed. Conclusion: These findings suggest that self-efficacy played an important role in dysphagia and it may affect the severity of dysphagia.