Social media health influencers have emerged as significant figures in the health information landscape of Nigeria, yet their specific influence on preventive health behaviour among youths in Delta State remains understudied. This study examined the influence of five selected Nigerian social media health influencers, namely Aproko Doctor, Doctor of the Future, Dr. Ayodele Renner, Dr. Bearded Sina, and Dr. Chioma Nmakama, on the preventive health behaviour of 400 youths in Delta State. Anchored on the Health Belief Model and using a descriptive survey design, data were collected through a structured online questionnaire administered via Facebook. The findings indicate that Aproko Doctor was the most recognised influencer, with 82.3% of respondents having either heard of or actively followed the account. TikTok emerged as the dominant platform for accessing health influencer content (65.3%), despite WhatsApp dominating general social media use (55.0%). Respondents reported broadly positive perceptions of influencer impact on disease prevention knowledge and lifestyle behaviour. Good personal hygiene (83.0%), healthy eating (76.0%), and adequate water intake (73.0%) were the most commonly adopted preventive practices. Message clarity (97.7%) and perceived medical qualification of the influencer (94.4%) were the strongest determinants of preventive health behaviour adoption. The researchers conclude that social media health influencers are most effective at producing lifestyle-level behavioural change, while clinical preventive behaviours such as vaccination and health screening remain constrained by structural barriers. Strategic partnerships between public health agencies and credible influencers, alongside investments in healthcare access, are recommended.