Despite substantial tobacco control measures, illicit cigarette exposure remains a persistent challenge among Malaysian youth, undermining public health and tobacco control efforts. This cross-sectional study investigated the behavioural, social, and environmental determinants of illicit cigarette exposure among 252 youths aged 15–30 years in Seberang Perai Tengah, Malaysia, using stratified cluster sampling. Pearson correlation and multiple linear regression analyses were employed to identify factors associated with exposure. Respondents reported moderate exposure to illicit cigarettes despite high awareness of smoking-related health risks. Accessibility, social normalisation, and peer influence emerged as significant positive predictors of illicit cigarette exposure, whereas perceived behavioural control and knowledge demonstrated significant protective effects. In contrast, health-risk perception, enforcement perception, and campaign effectiveness were not significant predictors. The final regression model explained 47.1% of the variance in illicit cigarette exposure, indicating that environmental and social influences outweigh cognitive awareness in explaining youth exposure. These findings contribute to youth studies by demonstrating that illicit cigarette exposure is shaped primarily by the interaction of environmental availability, peer dynamics, and social norms rather than knowledge alone. The findings further suggest that effective youth tobacco-control strategies should combine stronger regulation of illicit cigarette availability with peer-based prevention, community norm change, and interventions that strengthen behavioural self-regulation.