Purpose To determine the prevalence of professionally reported oral side effects of chemotherapy and the self-reported oral side effects and whether both prevalences could be related to the periodontal risk of the patients. Methods A cross-sectional study with patients undergoing chemotherapy treatment was carried out. Demographic, oral hygiene habits, and cancer-related data were collected while the patient was receiving the chemotherapy infusion. Patient’s oral status, measured according to the oral-assessment guide for patients in hospital environments, patient-related outcomes (PROMs), measured by a visual analogue scale, and patient’s periodontal risk were analyzed using validated questionnaires. Data was reported in means and standard deviations (SD) in quantitative variables and in counts, prevalence, and 95% confidence intervals (CI) in qualitative variables. ANOVA test and chi-squared tests were used to compare oral side effects among different periodontal risk groups. Results Three hundred sixty-nine patients were included in the study. The prevalence of professionally reported oral side effects was 86.99% (95% confidence interval CI 83.54%; 90.44%). The prevalence of self-reported oral side effects was 89.70% (95% CI 86.59; 92.82). The most common oral side effects were xerostomia (73.4%), dysgeusia (61.8%), and dry lips (54.2%). More oral alterations were found in patients with worse periodontal risk ( p  < 0.001). Conclusions The prevalence of oral side effects (professional or self-reported) is higher than 85% in patients undergoing chemotherapy. This prevalence increases as the risk of developing periodontal disease does.