Conspiracy theories have been widespread in public discourses about COVID-19. Belief in conspiracy theories has negative effects on COVID-19 protective and preventive behaviour. However, evidence on the prevalence of belief in COVID-19 conspiracy theories, remains fragmented. We conducted a systematic review on the adult general population prevalence of belief in different COVID-19 conspiracy theories at country and regional level around the globe. Searches were performed in PubMed, Scopus and Google Scholar databases (end of search: March 22, 2022). We followed guidelines for Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). We identified 1637 potentially relevant titles in our search and included 33 studies in our review, reflecting conspiracy theory endorsement between March 2020 and August 2021. We found high belief in COVID-19 conspiracy theories in most studies: 29 studies measured at least one conspiracy theory belief prevalence ≥20%. For several conspiracy theories, e.g. “Lab origin” or “Destabilization and power gain”, prevalence was often between 20 and 50%. Our findings call for increased awareness about COVID-19 conspiracy theories and the need for tailored, context specific conspiracy theory preparedness, prevention and control. Development and integration of a public health oriented communication and infodemic management strategy, alongside the implementation of conspiracy theory endorsement risk assessments are essential for supporting public health policy in future health emergencies.