Aim <p>Basic knowledge and understanding of the menstrual cycle physiology empower couples to realize reproductive plans: to conceive or postpone conception. On the other hand, risky sexual behaviors increase the chance of contracting sexually transmitted diseases. Adolescents indicate the internet and their peers as the most trusted source of information on sexuality. Many of them are involved in risky sexual behaviors. This study aimed to meticulously assess the knowledge about the menstrual cycle and contraception—its mechanism and risk associated—among the youth and examine their association with sexual behavior.</p> Subject and methods <p>An online, questionnaire-based cross-sectional study was conducted to assess the knowledge of the menstrual cycle physiology, contraception risks, and their relationship with risky sexual behavior among young people in Poland. The questionnaire included a total of 26 questions, both single- and multiple-choice questions, and was divided into three sections: 1. Sociodemographic data, 2. Knowledge about the menstrual cycle physiology and contraception risks, 3. Sexual behavior.</p> Results <p>Approximately two-thirds displayed general knowledge about the investigated matters; however, less than 50% could answer more specific questions correctly. Correctness of answers was associated with the age group 19–29 and living in a city with over 500,000 residents. Moreover, 67% admitted prior risky sexual behaviors, which were not significantly associated with alcohol consumption.</p> Conclusion <p>Further educational interventions are needed in the area, especially in smaller city centers. The findings of this study might be used as a baseline for further research.</p>