Background: Sexually transmitted infections (STIs) remain a public health concern due to their profound impact on sexual and reproductive health, particularly in developing countries. Many nations have investigated the root causes of STIs, however, there are limited studies published in the small island developing states in the Pacific, including Tonga. The lack of adequate information in the local context causes significant challenges to the proper planning and effective implementation of STIs prevention and control programs. Aim: This study aimed to assess the knowledge, attitudes, practices, and health seeking behaviours towards STIs among youth in Tonga, with a view to generating local evidence to strengthen preventive policies and targeted interventions. Methods: This cross-sectional study was conducted on a sample of 433 youth aged 15-24 years in Tonga. Interviews were conducted by trained interviewers using a standardised questionnaire that collected information on demographic characteristics, knowledge, attitudes, practices, health seeking behaviours towards STIs and the barriers to accessing sexual health services. The descriptive results and associations between variables were also presented. Results: Of the total youth survey, 233 (54%) were females, 277 (64%) were unemployed, and 399 (92%) were unmarried. Less than half of youth correctly identified HIV, chlamydia, and syphilis as STIs. The percentage of youth who correctly answered the routes of transmission and the symptoms of STIs were low. Infertility (51%) was the most recognised complication, and one third of youth correctly answered that cervical cancer, ectopic pregnancy, and stillbirth are some of the complications. Over 90% of youth perceived that condoms protect against the diseases; however, approximately 30% thought that the use of a condom is not required if both partners have symptoms of STIs and multiple sexual partners play no role in diseases transmission. Of the sexually active youth, only 47% used a condom the first time they had sex. The main reason was the limited availability of condoms. It was reported that 31% of females and 66% of males consumed alcohol, a contributor to risky sexual practices. Less than half of youth reported that they would access clinics if they had symptoms. The main barriers to seeking sexual health services were ‘being too embarrassed’ and issues with privacy at the clinics. Conclusion: This study highlighted the urgent need to strengthen national preventive policies and culturally acceptable targeted interventions to tackle STIs more effectively. The findings also provided a timely reminder to policymakers and health leaders to invest additional resources to effectively prevent and control; incorporate comprehensive and culturally acceptable sexual health education in schools; improve privacy, confidentiality, and accessibility of sexual health services in all relevant health care settings in Tonga.