Background: Sexually Transmitted Infections (STIs) can cause serious health complications, including pelvic inflammatory disease, infertility, ectopic pregnancy, adverse pregnancy outcomes, and cervical cancer. Adolescents and young adults are particularly vulnerable to STIs due to risk-taking behaviors, multiple sexual partners, and inconsistent condom use. Our study aimed to examine the knowledge about contraceptive methods and sexually transmitted infections, and attitudes toward sexual health education among adolescents in Croatia. Methods: The participants were high school students aged 16 to 18 years. The study was conducted at Antun Gustav Matoš Gymnasium, Zabok. We obtained study data via an anonymous survey. Results: The study included 204 adolescents in their final years of high school. The total number of adolescents who had sexual intercourse was 47 (23%). The percentage of sexually active female and male adolescents was 21% and 27%, respectively. In the surveyed group of adolescents, 86% had never heard of subcutaneous progestin implants and 72% had never heard of contraceptive sponges. All adolescents had heard of oral contraceptives and condoms. In the surveyed group of adolescents, 58% had never heard of trichomoniasis and 10% had never heard of HSV. All adolescents had heard of HIV and syphilis. A total of 147 (72%) of the surveyed adolescents believed it as necessary to provide health education in schools. Conclusion: The initiation of sexual activity among adolescents in Croatia remains unchanged when compared to previous studies. Their current knowledge of sexually transmitted diseases and contraceptive methods should be improved through continuous, well-designed, and comprehensive sexual health education.