Objective <p>This study aimed to assess the knowledge, perception, and preventive practices of Hepatitis B virus (HBV) infection among young adults in Lagos Island Local Government Area, Lagos, Nigeria.</p> Methods <p>A descriptive cross-sectional study was conducted among 371 young adults aged 18–35 years residing in Lagos Island Local Government Area. Participants were selected using a multistage random sampling technique. Data were collected using a structured, interviewer-administered questionnaire that assessed socio-demographic information, HBV-related knowledge, perception, and preventive practices. Data were analyzed using Epi Info 7.1, with descriptive statistics and chi-square tests applied to determine associations between variables. A p-value of &lt; 0.05 was considered statistically significant.</p> Results <p>Among respondents, 74.7% had heard of Hepatitis B, and 69% demonstrated good knowledge, although gaps remained concerning modes of transmission and complications. While 96.8% exhibited positive perceptions, such as willingness to be vaccinated or screened, actual preventive behavior was suboptimal: only 20.2% had been vaccinated, and 30.3% had undergone screening. Education level was significantly associated with knowledge, perception and preventive practices (<i>p</i> = 0.001). Additionally, knowledge was significantly associated with both perception and preventive practices (<i>p</i> &lt; 0.05). Barriers to preventive action included lack of awareness, time constraints, and limited access to vaccination services.</p> Conclusion <p>The study revealed that although awareness of Hepatitis B infection is high among young adults in Lagos Island, significant gaps remain in accurate knowledge and consistent preventive behavior. These findings contribute valuable insights for designing targeted, culturally grounded public health interventions aimed at improving Hepatitis B prevention among urban youth in Nigeria.</p>