A fast emerging area of public health interest is the potential role of social media influencers in spreading health information and affecting health behaviour. The aim of this research was to systematically synthesise evidence on the impact of social media influencers on health outcomes. A systematic search of six databases (Medline, Embase, PsycINFO, CINAHL, Science Citation Index and Sociology Collection) was conducted in October 2021, with an update search in January 2023. Two reviewers independently screened titles and abstracts. We included interventional study designs which examined the effect of social media influencer as the primary or only exposure on a quantified health outcome. We used narrative synthesis to summarise the characteristics and main findings of studies within each of four main topic areas. We assessed quality using a critical appraisal checklist for quasi-experimental studies. We identified twelve studies across four areas that met the inclusion criteria: children's dietary behaviour (n = 5), body image dissatisfaction (n = 4), influenza and COVID-19 (n = 2), and social comparison and anxiety (n = 1). Influencer marketing of unhealthy foods had a statistically significant effect on increasing children's immediate energy intake, but no effect was found for influencer promotion of vegetables. When an unhealthy food is marketed by an influencer of unhealthy appearance, this led to an increased preference for healthy snacks. Exposure to idealised influencer body imagery had statistically significant negative impacts on body image and mood. Anxiety caused by viewing idealised portrayals of motherhood was the same whether or not the portrayal was by an influencer. Public health campaigns harnessing influencers to promote hygiene habits in the context of COVID-19 and influenza showed some positive changes in health behaviours. Most of the published studies are of poor methodological quality with poor generalisability and statistical weaknesses. We conclude that social media influencers have both negative and positive impacts on health outcomes, with negative impacts seen consistently in studies of body image dissatisfaction. Further research is needed to harness the potential positive health impact of social media influencers, while mitigating against harmful effects.