An online sex education class was conducted for students of the National Institute of Technology, Oshima College using WebClass. This was a part of the institute’s efforts to impart education in the wake of the current COVID-19 pandemic. To measure students’ level of understanding, their sexual knowledge was assessed before and after the class. The results showed that students’ test scores after the class increased significantly compared to those before the class. There were differences in the percentage of questions answered correctly depending on the content of the test questions, which will be helpful in planning future sex education classes. Further, though only 40.5% of all students were able to attend a series of online sex education classes within the specified period, the learning effect of the online classes was commendable. Thus, it is necessary to improve the methods for getting more students take up online classes systematically.