Introduction: Oral cancer is a world health problem. It constitutes the sixth leading cause of cancer-related death. Knowledge about and awareness of this disease among the population is important to reduce its high mortality rate. Objective: Determine the level of knowledge about oral cancer among adult patients attending the Dental School at the National University of San Marcos in the year 2017. Methods: A cross-sectional observational descriptive study was conducted. The study population was the patients attending the Dental School clinic. The sample was 223 patients selected by systematic probabilistic random sampling. The evaluation tool was a survey containing 11 closed-ended questions divided into five domains: general knowledge, risk factors, signs and symptoms, repercussions and prevention of oral cancer. The grading scale was low, fair or high. Results: Of the patients surveyed, 56.5% (n= 126) had a low knowledge level, 40.4% (n= 90) a fair level and 3.1% (n= 7) a high level. Average answer level was 12.14 ± 2.90 (CI 95% [10.17-14.89]). Sex and age did not have a significant relationship to knowledge level (p= 0.45 and p= 0.52, respectively). However, educational level did have a significant relationship (p= 0.009). Conclusions: In the study population the level of knowledge about oral cancer is predominantly low. Sex and age do not have an influence on the level of knowledge about oral cancer, but educational level does.